The rotes of diffusion of CH4, n-CaHlo, iso-GHIo, and GF1o into well-characterized single crystals of sodium mordenite were studied a t 25" to 100°C., using a rapidly responding constant-volume system. The diffusion coefficients are of the order of lo-* to sq. cm/ sec. and exhibit o maximum value with time, probably due to an initial temperature increase caused by the heat of sorption. The implications to catalysis of these and other studies of diffusion in zeolites ore discussed.The widespread recent application of zeolites (molecular sieves) in selective adsorption processes and in catalysis has focused attention on the diffusion characteristics of molecules inside the tiny pores of these materials, for this can have a profound effect on dispersion in adsorption Irocesses and on reaction activity and selectivity in catalyis. The molecular sieve crystals may be bonded together o form pellets or, as in cracking catalysts, they may be ncorporated into a gel matrix. However, diffusion studies eported on these forms of molecular sieves may be diffiult to interpret because of the complex pore structure )resent and uncertainties concerning the effect of the )elleting process, use of binders, etc., on the ease of access d diffusing molecules into the pores of the zeolite. By tudying single crystals of uniform size, the above diffibulties can be avoided but the experimental limitations set )y apparatus constraints and the heterogeneous nature of ven single crystals must be clearly recognized in interpreting results. The present study used a rapidly responding, constant-volume sorption system but the results emphasize that initial temperature transients that can occur in such an experimental apparatus may substantially affect the results reported. The zeolite was a well-characterized sample of sodium mordenite. The two-dimensional pore structure characteristic of mordenites enhances the effect of structural heterogeneities on diffusion. Markedly different sor tion and diffusion characteristics may be can greatly affect their behavior in adsorption and catalytic processes.In an earlier study (11) the diffusion characteristics of the C1 to C4 paraffin hydrocarbon vapors into single crystals of sodium mordenite were investigated using a constant-pressure apparatus. The present study used a sample of the identical material previously used but in a different type of sorption apparatus that permitted accurate measurement of faster sorption processes. The sodiummordenite comprised single crystals of average dimensions 21 x 21 x 33 p , described previously in detail (11 ) . Some Taking the surface tension as 15 dynes/cm. and the molar volume as 31 ml. (2) and neglecting the thickness of the found for di P erent samples of synthetic mordenite, which adsorbed layer on walls of the pores, adsorption of krypton at P/Po above 0.016 would represent sorption into pores above 7 A. diameter. By this procedure 17% of the pore volume would be in pores above 7 A. diameter and 11% in pores above 13A. diameter; 6% of the total represents pore...