An insect-repellent composition is disclosed which includes an insect repellent such as p-menthane-3,8-diol and a synergistic amount of an acetal of formula (Z). A method of forming a preferred composition is also disclosed in which citronellal undergoes an acid-cyclization reaction to yield an impure mixture of p-menthane-3,8-diols and acetals. The impure mixture is subjected to fractional distillation to remove a low boiling point fraction and to thereby yield a remaining fraction which constitutes the preferred composition.