A suitable class of reagents for preparing the half-acid starch esters include substituted cyclic dicarboxylic acid anhydrides such as those described in U.S. Pat. No. 2,661,349 issued on Dec. 1, 1953 to Caldwell et al., having the structure ##STR4## where R is dimethylene or trimethylene and R' is a C8 -C18 alkyl or alkenyl group.