Disclosed are 3-aryl-2-cyano-3-hydroxy-acrylic acid derivates of formula(1a) (1b) or (1c) wherein R.sub.1 is hydrogen, C.sub.1-C.sub.20alkyl; C.sub.1-C.sub.20alkoxy; CF.sub.3; C.sub.6-C.sub.10aryl; or a radical of formula (1a) and R.sub.2 is hydrogen; or C.sub.1-C.sub.20alkyl. The compounds are used as antimicrobial actives for the treatment of surfaces. ##STR00001##