AbstractOsmotic pressure, initial phase separation, and viscosity results are presented for dilute solutions of cellulose nitrate, cellulose acetate, and ethyl cellulose, using polymers of similar molecular weight and degree of substitution. A fairly wide range of solvents has been used in each case. Osmotic pressure results suggest molecular dispersion of polymer in most solvents but there is evidence for association of ethyl cellulose in aromatic hydrocarbons. Values of the Huggins μ are obtained and, for systems involving cellulose nitrate and ethyl cellulose in homologous series of ketones and acetates, seem to fall to a minimum and then increase on ascent of each series. It is suggested that the heat term in μ should include an exothermic contribution arising from solvation in addition to an endothermic contribution resulting from mixing of solvated polymer and solvent. It is tentatively suggested that, in a homologous series of solvents, the entropy and exothermic contributions may not vary much and that the variation of μ may arise from varying endothermic contributions. The volumes of hexane required to cause initial phase separation of the nitrate and acetate from solution increase with solvent power, as estimated from μ, but with ethyl cellulose the volumes tend to decrease. It is suggested that solvated polymer may separate from solutions involving polar solvents and, making certain assumptions, estimates are made of the solubility parameters of the polymers separating. Those for cellulose nitrate vary markedly with solvent. The variation of solubility parameter with solvent is less with cellulose acetate and ethyl cellulose but in the latter case values for polymer separating from polar solvents are less than from nonpolar. Comparison of these estimated solubility parameters with those of the solvents and hexane suggests that the volumes of hexane required to cause initial phase separation will only serve as measures of solvent power if the solubility parameters of solvents and precipitant all lie on the same side of that of the polymer separating. This seems to be so for cellulose acetate and possibly the nitrate but not for ethyl cellulose. Comparison of viscosity parameters and values of μ suggests that in certain cases both [η] and the initial slope of the plot of ηsp/cagainstcincrease with solvent power. Other suggested viscosity measures of solvent power do not seem to be generally applicable. Some variation of chain configuration with solvent seems probable. Solvation, solvent type and association of polymer may also affect the viscosities of dilute solutions of cellulose deriv