Kinetic data are reported for the sorption of a range of aromatic sorb
ates in unaggregated crystals of NaX zeolite. Diffusivities (at 600 K)
range from order 10(-6)-10(-7) cm2.s-1 for benzene and p-xylene to 10
(-14)-10(-15) cm2.s1 for hexamethylbenzene and anthracene. The diffusi
vities decrease with increasing sorbate size, but the relationship is
complex and does not follow the sequence of either molecular weight or
critical molecular diameter. Rather the correlation appears to be wit
h the moment of inertia of the sorbate molecule, suggesting that restr
iction of the rotational freedom of the diffusing molecule is probably
a major factor. For several of the sorbates the validity of the diffu
sion model was confirmed by replicate measurements with different crys
tal size fractions.