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)alkyl; R4 is hydrogen or methyl; R5 is hydrogen or methyl; x is an integer from 1 to 2; y is 0, 1 or 2; with the proviso that when R3 is —B—(CH2CH2—O)z-R6, y is 1 and 5-(propargyloxy)-benzo[1,3]dioxole is excluded.