Experimental data are reported for adsorption of ethane and propane and their mixtures with water on BPL activated carbon. Pure alkane isotherms were measured at 273, 298, and 323 K from the Henry’s law region to approximately atmospheric pressure. Mixture data, including hysteresis measurements, were obtained at 298 K by fixing the water partial pressure (or relative humidity). Alkane adsorption is significantly affected by water, with loading reductions of up to 60% at a relative humidity of 50%. Our results show that alkanes do not adsorb independently of water, and greater loading reductions can be expected at higher relative humidities. Also, from our data we deduce that approximately 6% of the adsorbent surface strongly adsorbs water, probably on oxide sites. Pure alkane isotherms are analyzed in terms of a lattice model, and a simple empirical method is proposed for calculating mixture equilibria from pure-component isotherms. The method is applied to our data as well as to data for other organic-water mixtures adsorbed on BPL carbon. Results are compared with predictions of previously published methods.