Considering the fact that a high level of protection by butyl rubber gloves or coveralls against chemicals is often achieved at the expense of significant thermal discomfort and perspiration, an investigation has been performed to assess the ability of butyl rubber composite barriers to sustain their functional and structural integrity in the presence of high humidity and heat. Carbon black-reinforced butyl rubber barriers absorb a substantial amount of moisture, with the hygroscopic reinforcement phase making a more significant contribution than the matrix. The exposure to moist heat up to 70°C resulted in the decrease of the tensile modulus (uniaxial as well as biaxial) and the increase of tensile failure strain of composites. The observed softening effect was reversible upon drying. On the other hand, the exposure to dry heat at the same temperatures tended to stiffen the composites, presumably because of the loss of compounding additives, post-curing, or chain scission. The same process of heat aging is supposed to occur in moist heat as well. Under the proven assumption that the effect of moist heat is a superposition of moisture absorption effect and dry heat effect, the extent of moisture-induced softening of composites was estimated by subtracting the mechanical property changes in moist heat from those in dry heat. The activation energy for moisture-induced softening was found to be the same as that for moisture diffusion of the composites and for moisture absorption of carbon black phase, supporting a hypothesis that moisture absorption of carbon black phase softens the composites via weakening of the secondary bonding between the reinforcement and the matrix.