SummaryAt relatively low temperatures boron trifluoride and hydrogen fluoride gave high yields of polymers from soybean fatty acids and methyl esters.The use of 2% boron trifluoride as catalyst at 150° to 200° resulted in the polymerization of 50–60% of the methyl esters within one hour. The viscous polymeric fraction usually had a dark color, a high acid number, and a low ratio of dimer to higher polymer.The boron trifluoride‐ether complexes were equally effective catalysts and more convenient to handle. Use of the aliphatic ether complexes resulted in products having lighter colors.It was necessary to use large amounts of hydrogen fluoride to produce comparable yields at less than 100°, but 70% yields of polymers having light colors and low acid numbers were obtained.