Commercial cornstarch was mono-phosphorylated to different levels of substitution in order to investigate the effect of phosphorylation on the properties of cornstarch for sizing heat-sensitive wool yarns at reduced temperature. The influences of starch phosphorylation and sizing temperature upon apparent viscosity and viscosity stability of cooked starch paste, starch retrogradation, adhesion to wool fibers, performance of starch film, aerobic biodegradation, mechanical properties, and hairiness of sized wool yarns were evaluated. The phosphorylation level was varied from 0.021 to 0.082 in degree of substitution (DS), while the temperature considered was from 60 to 95 degrees C. It was found that mono-phosphorylation of starch resulted in enhanced paste stability, reduced retrogradation, strong adhesion to wool fibers, increased performances of starch film, improved mechanical properties of sized wool yarns, and decreased hairiness on surface of sized yarns even if paste temperature was lowered to 60 degrees C. Initially increasing phosphorylation level enhanced positive effects, but excessively increasing the level was not applicable due to marked reduction in tensile strength of starch film. The phosphorylation with a DS value of 0.061 could improve the performances of cornstarch for sizing wool yarns at 60-80 degrees C. Moreover, measurement on BOD5/COD ratios demonstrated that the phosphorylation did not impede aerobic biodegradation of starch. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2012