The pyrolysis of 25-(1-methylsulphinylalkyl)avermectins provides, in good yield, novel avermectins with C-25 substituents containing terminal alkenyl groups. The use of palladium(II) acetate catalyst permits the highly selective elaboration of such compounds to more complex C-25 alkenes and their derivatives, without the use of protecting groups and in the presence of acid- and base-labile functionalities.