The influence has been studied of both the acid strength of phosphoric acid and the degree of substitution of cellulose acetate on the formation of an anisotropic phase. The solvent composition is expressed as a P2O5 concentration. It was found that the clearing temperature increases strongly with decreasing amount of water in the solvent. The influence of the degree of substitution (DS) was studied on a series of samples with a DS ranging from 0.23 to 2.89. It was found that over the entire range good solubility occurred in anhydrous phosphoric acid. The DS hardly affected the clearing temperature in the process of dissolving a constant number density of polymer chains. However, the birefringence of the solution was strongly influenced by the DS, due to the intrinsic birefringence of the polymer chains.