Permeation measurements indicate that a SSZ-13 membrane exhibited significantly different adsorption behavior from SSZ-13 crystals. Both C(3)H(8 )and n-C(4)H(10 )adsorb to high loadings in SSZ-13 crystals, but they only have a small effect on CO2 permeation through a SSZ-13 membrane at 293 K, indicating they do not adsorb in the SSZ-13 pores. Whereas 15% C2H6 in the membrane feed decreased CO2 permeance by 45%, 15% C3H8 only decreased CO2 permeance by 10%, even though C3H8 adsorbs more strongly than C2H6 in SSZ-13 crystals. Moreover, this 10% decrease is attributed to C3H8 adsorption on the external surface of the membrane crystals because i-C4H10, which is significantly larger than SSZ-13 pores and thus does not adsorb in the pores, only decreased CO2 permeance by 15%. The difference between SSZ-13 crystals and the SSZ-13 membrane is attributed to the presence of a diffusion barrier on the external surface of the membrane crystals. Thus, care should be taken in using adsorption measurements on zeolite powders to predict permeation behavior in zeolite membranes.