Oxidized starch powder and cationic starch solution were reacted with alkyl ketene dimer (AKD) under heterogeneous conditions at 70℃ for 12 h. The AKD molecules reacted with starch hydroxyl groups to form β-keto ester linkages under the above conditions. The reaction products were separated into CHCl₃-soluble and CHCl₃-insoluble fractions. FT-IR spectroscopy, SEM, Xray diffraction, and TG-DTA analyses of the CHCl₃-insoluble fraction indicated that β-keto ester substituents were introduced to hydroxyl groups on the starch surfaces. The results indicated that hydrogen bonds between the starch molecules were disrupted under heterogeneous conditions upon heating. The activity and accessibility of hydroxyl groups were enhanced, as a result of which β-keto ester bonds were produced between AKD and starch. Based on these results, we speculate that the β-keto esters which existed in the sheets sized by AKD emulsions were generated by the reaction between AKD and starch.