Guar gum is a naturally occurring galactomannan, which is extremely hydrophilic in nature. Hydroxypropyl guar gum (HPG) was subjected to acetylation using acetic anhydride and pyridine. The effect of the reduction in the hydrophilic nature of the polymer on its filler properties was studied by using the derivatized guar gum as filler in an unsaturated polyester composite. The effect of degree of substitution and the concentration of filler on the rheological, chemical, and mechanical properties of the composites were studied. Results indicated that an increase in the acetate content in the HPG resulted in an increased polymer filler interaction. However, the composites resulting from these derivatized HPGs showed reduced mechanical properties. This decrease in the mechanical properties were attributed to the decrease in the hydrogen bonding in the filler particles, thus reducing the cohesiveness and strength of the filler particles themselves. Thus, polysaccharides can be used as fillers but chemically modifying them results in a change in the basic nature of the filler itself and is not just restricted to surface modification as is the case of inorganic fillers and fibers. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 3204-3210, 2010