Abstract An investigation was made of the effect of the nature of elastomer, thickness of the membrane and its formulation, and the temperature and relative humidity of the surroundings, on the permeation of bis(2-chloroethyl)sulfide (SM), a well-known chemical warfare agent, through solution-cast elastomer membranes. The permeation was evaluated in terms of the breakthrough time of the agent through the membrane, using the spot disk test and the weight gain method. Butyl rubber exhibited the highest permeation resistance, which increased linearly with thickness and was increased by incorporation of an optimum of 20 phr carbon black in the membranes. Increase in temperature and relative humidity lowered the permeation resistance. The observed data were correlated with permeation parameters and it is postulated that permeation of SM through elastomers is non-Fickian in nature.