A composition comprising an alkynyl phenoxy compound of Formula (I) as a synergist and a pesticidal active ingredient is described wherein R1 and R2 similar or different are (C1-C4)alkyl or R1O- and R2O- together represent a group -O-CH2-O- - O-CH(CH3)-O -O-CH2-CH2-O- -O-CH2-CH2- R3 is (C1-C6)alkyl(C3-C6)alkenyl or B-(CH2-CH2-O)z-R6 where B is -CH2- O- or O- z is 0 1 or 2 and R6 is (C1-C4)alkylR4 is hydrogen or methylR5 is hydrogen or methylx is an integer from 1 to 2 y is 0 1 or 2 with the proviso that when R3 is B-(CH2-CH2-O)z-R6 y is 1 and 5-(propargyloxy)-benzo 1 3 dioxole is excluded.