The sensitivity of twenty six microbial lipases towards acetaldehyde (an unavoidable by-product in lipase-catalysed acyl transfer reactions with vinyl esters) was investigated. The sensitivity of an individual enzyme strongly depends on its properties such as microbial source, molecular weight and relative lysine content. Whereas the majority of enzymes (from Pseudomonas, Rhizopus, Chromobacterium, Mucor and Candida antarctica sp.) proved to be remarkably stable, lipases from Candida rugosa and Geotrichum candidum lost most of their activity when exposed to acetaldehyde.