Phase diagrams for the system nitrocellulose/tetrahydrofuran/ethanol have been mapped out at room temperature, for two different degrees of substitution of the polymer (2·32 and 2·72). The phases formed by particular compositions were identified by using polarized light microscopy. There is a range of compositions which separate into an isotropic phase and a liquid-crystalline phase; the concentrations involved are qualitatively in agreement with Flory's theory of phase separation in a solution of rigid rod-like molecules. Our plotted room temperature sections of the phase diagrams obey the Gibbs phase rule at all points. We report evidence for the formation of a crystallosolvate of nitrocellulose (degree of substitution 2·72) and ethanol. The crystallosolvate formation only appears to be kinetically possible if there is initially also sufficient tetrahydrofuran present to dissolve the polymer.