Flat sheet homogeneous CA membranes of different thicknesses and different acetyl contents (43.1, 39.1, 32.0 wt.-%) were cast from solutions of Bayer Cellit T-900 and K-700 as well as Eastman Kodak CA E-320 In mixtures of dioxan with dimethylacetamlde (9:l by volume), in pure methyl glycol acetate, and in mixtures of methyl glycol with either dimethylformamide or dimethylacetamide (9:l by volume). The permeabllities (Pg), diffusion coefficients (Dg), and solubilities (Sg) of the membranes for He, Np, Cl%,, and CO2 were then determined as functions of pressure (0.1 atm to 100 atm) and temperature (2@C up to SU'C) by use of time-lag measurements. In the presence of pin-holes the permeabilities increased linearly with increasing mean pressure whereas imperfection-free membranes exhibited both pressure-independent and slightly pressure-dependent permeabilities, diffusivities, and solubilities. The sorption isotherms of He, N2, and Cl%+ obeyed either Henry's law or the dual-sorption model relationship whereas the sorption isotherm of CO2 followed Henry's law only up to pressures of p' = 18 atm and increased nonlinearly with increasing p' at pressures beyond P ' 2: 18 atm. The temperature dependence of Pg, Dg, and S I' measured at some reference pressure, yielded activation energies of permeat on, EP, and diffusion, BD, as well as enthalpies of sorption, AH,.