Sorption and diffusion of C4 and C5 hydrocarbons have been studied in thin films of ethyl cellulose in the temperature range 30 to 80°C. Only regions of low penetrant concentration were chosen in order to minimize hysteresis effects associated with slow relaxation of polymer chains. The data have been quantitatively compared with those for the same penetrants in rubber, since ethyl cellulose and rubber are examples of stiff and flexible chain polymers, respectively. In both solution and diffusion some major differences were found between the two types of medium. In rubber the heats of dilution were small and the corresponding positive entropies close to those predicted by the lattice theory of solutions. On the other hand, in ethyl cellulose at low penetrant concentrations the heats of dilution were strongly exothermal and the entropies negative. For both polymers diffusion coefficients of penetrants were dependent upon concentration, but whereas in rubber this concentration dependent decreased rapidly with rising temperature, in ethyl cellulose it was largely independent of temperature. Energy barriers involved in each unit diffusion process were comparable for a given hydrocarbon in rubber and in ethyl cellulose, while the molecular shape of the penetrant molecule substantially influenced the diffusion coefficients in each polymer. Further interpretation of the diffusion coefficients suggested that the involvement of polymer chains in each unit diffusion is much less for ethyl cellulose than for rubber. Explanations are offered, based upon the nature of the chains composing the two media and upon the existence of holes in the ethyl cellulose network, for the essential differences indicated above. The relative efficiency of ethyl cellulose and rubber membranes for gas separation is discussed.