Abstract Studies of the swelling of rubber which had been carried out up to 1924 were reviewed by one of the authors in an earlier publication. A review of subsequent studies is to be found in a paper by Scott, who himself has contributed some data on the subject. These prior studies were not, however, concerned primarily to secure systematic information on the influence of the chemical structure of liquids on their imbibition by rubber, and they involved only a relatively small number of liquids, except for the study by Lee, in which the behavior of raw rubber in sixty-nine liquids was observed only qualitatively. Further, a good many of the earlier observers used material of unknown history and composition, such as “black rubber tubing”, and often assuming the swelling to reach a maximum in twenty-four hours. In the present study the course of the swelling of rubber, both raw and vulcanized, has been followed, quantitatively, at intervals over long periods, and a very large number of liquids have been examined. The present installment deals largely with classes of liquids (acids, alcohols, aldehydes, ketones, etc.) containing the more highly polar groups. A collection of hydrocarbons is, however, included, because hydrocarbons are in general good swelling agents, and data on them will serve for comparison with that on other classes of compounds.