Abstract Since we found no appreciable reaction between phthalic anhydride and cis-polyisoprene at 140°C, it is very unlikely that the scorch delay resulting from the use of phthalic anhydride is due to the blocking of cure sites of diene rubbers by phthalic anhydride. Rather, we incline to believe that the formation of a PA-ZnO-MBT complex during the cure is responsible for the observed scorch delay. “Starvation” of zinc oxide by phthalic anhydride was considered as an alternative explanation for the scorch delay, but it was ruled out on the basis of the finding that neither 0.8 phr of additional zinc oxide nor the reaction product of phthalic anhydride with zinc oxide showed very much effect on scorch delay.