There are numerous reports in the literature indicating that water vapor may interfere with the adsorption of hydrocarbons on activated carbon adsorbents. This effect has not been considered important in the design of evaporative loss control devices (ELCD) to date. As most automobiles operate in an environment where the relative humidity averages 60%, it was suspected that humidity might have an effect on ELCD performance.Using the Cyclic Evaporative Loss Test Instrument Concept (CELTIC), the authors have previously shown that high relative humidity has an effect on canister performance under controlled laboratory conditions. Other investigators have noted regional effects on SHED performance which might be ascribed to high relative humidity.This paper will present results obtained employing the CELTIC system to simulate the conditions used by others. The results demonstrate that humidity can account for their results. In addition, the CELTIC system was employed to investigate the effect of water vapor on the composition and sequence in which hydrocarbons are emitted from an activated carbon ELCD canister. The results indicate that the low molecular weight butanes are emitted from the canister first and are displaced by progressively higher molecular weight hydrocarbons.These data were used to estimate the point at which the SHED test limits would be exceeded.