AbstractThe osmotic pressures of dilute solutions of 12.2%‐nitrogen‐content cellulose nitrate have been determined, with homologous series of methyl ketones and alkyl acetates as solvents. Values of the interaction constant μ have been estimated from the slopes of plots of reduced osmotic pressure against concentration. The order of solvent power obtained from values of μ is compared with the orders given by suggested estimates of solvent power from parallel precipitation and viscosity studies. The volumes of hexane required for initial phase‐separation from solution appear to be at least semi‐quantitative measures of solvent power as estimated from values of μ. Limiting viscosity numbers and the initial slopes of plots of ηsp./c against c can, to a less extent, be related to solvent power, but the relative stiffness of the chains seems responsible for the absence of any obvious relationship between solvent power and either the Huggins k' value or the Spurlin slope of Martin plot divided by limiting viscosity number.