It is necessary to create very specific and sensitive methods for assaying formaldehyde and methanol which are produced on the large scale and are very toxic and have mutagenic and carcinogenic action on living organisms. The methods for determination of formaldehyde, methanol and ethanol in the environment and fermentation products published and developed by the authors are reviewed in this paper. Most of the known methods are not sufficiently selective and sensitive and some of them are very expensive. Classical chemical, enzymatic, chemosensor and biosensor approaches used for methanol and formaldehyde assay are described. Enzymatic methods exploiting alcohol oxidase isolated from the mutant over-producing strain of methylotrophic yeast Hansenula polymorpha permit efficient determination of formaldehyde in industrial wastewaters. Enzymatic-chemical method based on the use of alcohol oxidase and 4-amino-5-hydrazine-3-mercapto-1,2,4-triazole (AHMT) allows simultaneous determination of methanol and formaldehyde. The technology of biosensor construction and their bioanalytical characteristics are described. Experimental data concerning amperometric and potentiometric biosensors based on the use of genetically modified cells of methylotrophic yeast Hansenula polymorpha are reviewed. The possibility to use alcohol oxidase-based biosensor for the assay of methanol in wastewater is demonstrated.