Experimental zero length column (ZLC) response curves are presented for benzene and p-xylene in silicalite over a range of conditions under which the isotherm varies from nearly linear to highly favorable. It is shown that the analytic approximation suggested by Brandani (Chemical Engineering Science, 53, 1998, 2791-2798) provides a good representation of the experimental curves and that this model allows the extraction of reliable values for the diffusional time constant even under non-linear conditions. The conditions under which it is possible to use the simplified linear analysis without introducing major errors are also examined.