Careful viscosity and osmotic pressure measurements with a membrane of known reliability are reported for 14 fractions of cellulose acetate prepared by repeated refractionation, For molecular weights up to 115,000 the following relation is obtained between intrinsic viscosity and absolute molecular weight: the units of [η] being 100ml./g. An explanation is offered for the difference between this equation and other equations in the literature as well as for the discrepancies among the various literature equations.

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Osmotic pressure measurements are reported for solutions of unfractionated and fractionated cellulose acetate in acetone, using denitrated collodion, gel‐cellophane, and caustic‐treated cellophane as semipermeable membranes. Although techniques are employed that are presumably correct for the effect of diffusion, different osmotic pres‐ sures and hence apparent molecular weights are obtained with the three types of membranes. No correlation is found between osmotic pressure and either the permeability constant or the average pore diameter of the membranes. Measurements with a very low molecular weight cellulose acetate, the molecular weight of which is known from independent measurements, show that the rate of diffusion of small solute molecules through various membranes is not related to the rate of transfer of solvent molecules.It appears that the observed differences between membranes are caused by rapid diffusion of low molecular weight material at the beginning of the osmotic measurement, before even the first readings by a dynamic method can be taken. From measurements with the very low molecular weight cellulose acetate it is concluded that, in the case of cellulose acetate in acetone, correct osmotic molecular weights are obtained only with undried gel‐cellophane membranes, and that molecular weights determined with either the faster denitrated collodion or the slower caustic‐treated cellophane membranes are appreciably too high.