AbstractIn order to establish the nature of the reaction by which the dextran molecule is formed we have examined the intensity of scattered light at eleven angles during the polymerization of sucrose in the presence of purified dextransucrase from B‐512‐F Leuconostoc mesenteroides. The molecular weight increases from about 50 × 106 to 100–175 × 106 at complete sucrose conversion, and then continues to increase to over 500 × 106. It is shown that the molecules are highly branched early in the reaction and become even more so as the reaction proceeds. These results are believed to be due to the formation of random, long‐chain branches by rearrangement of linear chains under the influence of a branching enzyme which is more heat‐stable than the polymerizing enzyme.