The carboxymethylation of granular potato starch by sodium monochloro acetate (SMCA) in i-propanol - water mixtures was studied with experimental design. The optimal reactions conditions were determined for the three quantitative responses, the degree of substitution of the starch hydroxyl groups by ether groups, the conversion of SMCA and the selectivity of the reaction of SMCA to carboxymethylated starch, and for one qualitative response, the product quality. The regions for the temperature, the water fraction and the amount of starch where the modified starch keeps it granular shape have been identified simultaneously. A kinetic model was used successfully to describe the time dependency of the quantitative responses.