When starch is digested in hot alkali the aldehydic terminal groups on the starch molecules are subjected to an enediol rearrangement to give organic (usually acetic- and lactic-) acids. A slow progressive destruction of the starch chains follows. These acids are determined by measurement of the amount of alkali, expressed as mls of 0.1N Sodium hydroxide, consumed by a given weight of starch when treated under defined conditions. This is the alkali number and is a measure of the number average molecular weight of the starch in the original untreated state. The method is applied to native starches, acid thinned starches, dextrins, and enzyme modified starches.