Chitosan (Ch)(deacetylation degree, 59-86%), 1-5% in aq. 1 % AcOH, mixed with aq. solution of (A) poly(vinyl alcohol) (M = 7.2-10(4)), (B) poly(ethylene oxide) 1-10(4), (C) potato starch, or (D) hydroxypropyl cellulose (M = 105) gave homogeneous mixtures. At C > 40%, Ch-C films spread on glass were heterogeneous. So were Ch-B films, Ch-chains built into B-spherolites were compatible in the amorphous phase. Films Ch-A and Ch-D were homogeneous over the whole concentration range, demonstrating compatibility in the solid phase. The H-bonds formed by residual H2O affected phase transitions. Viscosity curves of Ch-A, Ch-C and Ch-D showed neg. deviations from additivity. Degradation by UV disrupted glycoside bonds and eliminated amine groups. With Ch-A and Ch-B, membranes of controllable pore size were prepared, whereas Ch-A, Ch-C and Ch-D proved good to impregnate paper. The degree of deacetylation and the wt. fraction of chitosan in mixtures affected chitosan properties and stability.