The haem monooxygenase cytochrome P450(cam) has been engineered to oxidise the gaseous alkanes butane and propane to butan-2-ol and propan-2-ol, respectively, by the use of bulky amino acid substitutions to reduce the volume of the substrate pocket and thus improve the enzyme-substrate fit: the F87W/Y96F/T101L/V247L mutant oxidises butane with a turnover rate of 750 min(-1) and 95% yield based on NADH consumed while the wild-type enzyme has an activity of 0.4 min(-1) with 4% yield.