Butyl rubber (IIR) membranes containing a 0-50% volume fraction of carbon black(GPF N660) or nonblack fillers (talc/precipitated CaCO3/fullers earth) were prepared by solution casting and evaluated for permeation resistance to the vesicant, bis(2-chloroethyl) sulfide, or sulfur mustard (SM) and its oxygen analog (OA), bis(2-chloroethyl)ether. The diffusivity of the chemicals was determined from the breakthrough time (BTT), obtained using the spot disc test; and a gravimetric method. The permeation parameters, calculated from the sorption data, were used to determine an optimum filler content for maximum protection against SM. To investigate the effect of carbon loading on the permeation properties of IIR, the permeation of SM and OA in carbon-loaded black membranes was compared with nonblack membranes and interpreted in terms of the dispersion of carbon in the membranes.