An improved process for the sulfonation of an unsaturated elastomeric polymer to form a sulfonated elastomeric polymer, wherein a cement which contains an elastomeric polymer dissolved in a nonreactive solvent is contacted with a sulfonating agent at between about -100.degree. C. and +100.degree. C. for a period of time sufficient to result in sulfonation of the elastomeric polymer wherein the improvement includes the use of a sulfonating agent which is formed from mixtures of acyl sulfate ##STR1## with carboxylic acid anhydrides ##STR2## with carboxylic acid halides ##STR3## or with both, wherein R is selected from the group consisting of CH.sub.3 --, C.sub.2 H.sub.5 --, C.sub.3 H.sub.7 --, C.sub.4 H.sub.9 --, C.sub.5 H.sub.11, C.sub.6 H.sub.13 or C.sub.6 H.sub.5 and mixtures thereof. The molar ratio of ##STR4## is at least one.