Electrophoretic analysis of alcohol oxidase purified from the methylotrophic thermo- and acidotolerant yeast Hansenula sp. revealed the presence of two active forms of the enzyme with molar mass 440 kg/mol (major component) and 724 kg/mol (minor component). A subunit M of the enzyme was found to be 72 kg/mol. Two active forms of the enzyme found by electrophoresis seem to be caused by dissociation of the octameric form to the tetramer under alkaline conditions. Studies of alcohol oxidase showed a kinetic variability of the enzyme with respect to its Km. It is proposed that the variability of Km is caused by enzyme binding to formaldehyde.