delivered to customer sites as a result of an increase in the number of hydrogen installations completed under GenKey agreements and higher fuel costs. Gross margin increased to (22.9)% during the three months ended June 30, 2020, compared to (24.9)% during the three months ended June 30, 2019 primarily due to an increase in the number of sites, improved efficiencies on existing hydrogen sites, decrease in cost of fuel paid to suppliers, all of which was offset by the amount of provision for common stock warrants.
Cost of revenue from fuel delivered to customers for the six months ended June 30, 2020 increased 7.9%, or $1.3 million, to $18.1 million from $16.8 million for the six months ended June 30, 2019. The increase was primarily due to higher volume of hydrogen delivered to customer sites as a result of an increase in the number of hydrogen installations completed under GenKey agreements and higher fuel costs. Gross margin decreased to (23.1)% during the six months ended June 30, 2020, compared to (22.7)% during the six months ended June 30, 2019, primarily due to increase in the amount of provision for common stock warrants. The provision for common stock warrants from fuel delivered to customers for the six months ended June 30, 2020 and 2019 had a 9.7% and 4.1% negative impact on revenue, respectively, and was partly offset by improved efficiencies on existing hydrogen sites.
Expenses
Research and development expense. Research and development expense includes: materials to build development and prototype units, cash and non-cash compensation and benefits for the engineering and related staff, expenses for contract engineers, fees paid to consultants for services provided, materials and supplies consumed, facility related costs such as computer and network services, and other general overhead costs associated with our research and development activities.
Research and development expense for the three months ended June 30, 2020 increased $0.8 million, or 9.2%, to $9.8 million, from $8.9 million for the three months ended June 30, 2019. The increase was primarily due to additional R&D program investments such as programs associated with improvement of fuel efficiency, GenDrive unit performance and new product development such as on-road delivery trucks, drone applications, and increase in headcount.
Research and development expense for the six months ended June 30, 2020 increased $3.9 million, or 23.7%, to $20.2 million, from $16.3 million for the six months ended June 30, 2019. The increase was primarily due to additional R&D program investments such as programs associated with improvement of fuel efficiency, GenDrive unit performance and new product development such as on-road delivery trucks, drone applications, and increase in headcount.
Selling, general and administrative expenses. Selling, general and administrative expenses includes cash and non-cash compensation, benefits, amortization of intangible assets and related costs in support of our general corporate functions, including general management, finance and accounting, human resources, selling and marketing, information technology and legal services.
Selling, general and administrative expenses for the three months ended June 30, 2020, increased $8.0 million, or 58.9%, to $21.7 million from $13.6 million for the three months ended June 30, 2019. This increase was primarily related to acquisition and debt restructuring charges in addition to increases in salaries, employee bonuses, stock-based compensation and headcount.
Selling, general and administrative expenses for the six months ended June 30, 2020, increased $9.7 million, or 42.4%, to $32.7 million from $23.0 million for the six months ended June 30, 2019. This increase was primarily related to acquisition and debt restructuring charges in addition to increases in salaries, employee bonuses, stock-based compensation and headcount.
Interest and other expense, net. Interest and other expense, net consists of interest and other expenses related to our long-term debt, convertible senior notes, obligations under finance leases and our finance obligations, as well as foreign currency exchange losses, offset by interest and other income consisting primarily of interest earned on our cash and cash equivalents, restricted cash, foreign currency exchange gains and other income. Since June 30, 2019, the Company assumed approximately $120.0 million of additional long-term debt at 12% interest (which interest was reduced to 9.5%