Mixtures of benzene/xylenol gelled by cellulose acetate were much used as incendiary materials, and the present study of these systems was undertaken to elucidate the problem of syneresis on storage. This involved a study of the phase behaviour of the commercial cellulose acetate (heterogeneous both as regards chain length and acetyl number), in binary mixtures of xylenol (solvent) and benzene (non-solvent). A fixed concentration (4% by weight) of cellulose acetate in various benzene/xylenol mixtures ranging from pure benzene to 17% xylenol (by weight) was used, and the composition of the liquid and swollen phases in equilibrium found by analysis after centrifuging. The cellulose acetate in each phase was characterized by both chain length and acetyl content. In this way it was shown that the commercial cellulose acetate contained three major fractions, which differed but little in average chain length, but to a great extent in acetyl content. The three major fractions were then separated in sufficient quantity to enable their separate phase behaviour to be determined. It was found that the solubility of any one fraction was affected by the presence of the others, and an explanation for this has been advanced.