Phanerochaete chrysosporium degraded aspen wood lignin as well in 2 atm O2 (1 atm = 101.325 kPa) as in 1 atm O2, but 3 atm O2 inhibited the fungus, and O2 pressures above 4 atm killed it. Lignin degradation in 5 atm of air was similar to that in 1 atm of O2, indicating that O2 concentration, not pressure, was the inhibitory factor. The selectivity with which P. chrysosporium metabolised lignin in preference to other wood components did not increase at O2 pressures above 1 atm.