The half-lives (tin1/2) of acetate sex pheromones andn-alkyl acetates were determined on either or both West Co. and Arthur H. Thomas Co. natural rubber septa. These septa gave equal half-lives. Thet _1/2 values covered a large range: e.g.,t _1/2 for decyl acetate was 5 days and that for pentadecyl acetate was 1353 days. The expected linear relationship between Int _1/2 versus the number of carbon atoms was found for the six acetates fromn-decyl to pentadecyl, but thet _1/2 of hexadecyl acetate was only 478 days. This nonlinearity in the plot is attributed to the presence of polymer cross-links which create molecular size cages and produce an effect similar to that produced in gel permeation chromatography wherein large molecules elute faster than smaller ones. Thet _1/2 of 4 monounsaturated acetates were close to thet _1/2 for the corresponding saturated acetates, but this relationship may not be general. The half-life of one nonconjugated diunsaturated acetate was much less than that for the corresponding monounsaturated acetate, whereas the half-life of another nonconjugated diunsaturated acetate was similar to the half-life of the corresponding monounsaturated acetate. Thet _1/2 values determined for the pheromones may be used to estimate evaporation rates, the length of time the evaporation rates will be within a given range, and, with multicomponent pheromones, the ratio of components in the vapor.