The flow properties of solutions of derivatives of cellulose and of poly(vinyl alcohol) (PVA) formed by reaction of the polymers with a reactive dye, Cibacron Brillant Red 3B-A, have been examined. In both derivatives a very high degree of substitution has been achieved. The result is that the solutions show characteristic optical properties of liquid crystal solutions, being anisotropic at high concentrations. The theological behaviour is also typical of liquid crystal solutions. In the cellulose derivative the viscosity fell between 35% and 40% concentration and rose, for the 40% solution, between 40 and 50 degrees C. Time-dependent viscosity increases in the solution on resting following shearing were found to be paralled by changes in structure observed optically. In the solutions of PVA, colour changes could be observed at higher concentrations and the flow behaviour showed anomalous changes with concentration. The flow behaviour of both derivatives can be explained by the dye molecules forcing the semirigid cellulose and the flexible PVA chains into a rigid rod-like conformation, thus enabling them to form liquid crystal structures in their solutions.