A phenomenon was observed after exposing a fresh bed of activated carbon to a flow of natural gas that included several percent of ethane and propane in addition to the bulk methane. For several hours the temperature of the natural gas exiting the carbon bed was about higher than the temperature of the gas entering the bed. This phenomenon can be attributed to release of the latent heat of vaporization due to condensation of all of ethane and propane into the pores of carbon. Due to its hydrophobic nature carbon is not expected to produce significant condensation of water vapor. The calculated additional water vapor condensation heat release and its related temperature increase was not observed.