For the first time, carboxymethyl starch (CMS) samples with a very high degree of substitution (DSCM = 2.1) were synthesized by multistep carboxymethylation under heterogeneous reaction conditions in methanol/water with sodium hydroxide and monochloroacetic acid as an etherifying agent. The stepwise increase in the total DSCM value gradually decreased with an increasing DSCM value of the starting polymer. The determination of the functionalization pattern of CMS by H-1-NMR spectroscopy after chain degradation indicated a high preference for 2-O-substitution. The distribution of the carboxymethyl functions was in the order O-2 much greater than O-6 > O-3. A detailed analysis of the depolymerized sample by means of high-performance liquid chromatography and capillary electrophoresis revealed a monomer composition that was in very good agreement with the statistical model of Spurlin.