A compound of formula I ##STR1## in which formula: R.sub.a represents hydrogen or a C.sub.1 -C.sub.6 alkyl group, and R.sub.c represents a C.sub.1 -C.sub.6 alkyl group;R.sub.b represents a phenyl group, optionally substituted by a C.sub.1 -C.sub.6 alkyl group or halogen, a group of formula --CH.dbd.CR.sub.d R.sub.e in which R.sub.d and R.sub.e, which may be identical or different represent hydrogen or C.sub.1 -C.sub.6 alkyl groups or a group of formula --CH.dbd.NOR.sub.f in which R.sub.f represents hydrogen or a C.sub.1 -C.sub.6 alkyl group andRCOO represents a residue of an acid RCO.sub.2 H, which acid, or an ester-forming derivative of which acid, on reaction with .alpha.-cyano-3-phenoxybenzyl alcohol or an ester forming derivative thereof, gives rise to an .alpha.-cyano-3-phenoxybenzyl ester having pesticidal properties.