The results of electric-breakdown tests with direct voltage on transformer oil containing additives known as hydrogen acceptors are given in the paper. The test samples were filtered and carefully degassed in a closed test apparatus with an incorporated test cell. The samples were also stress-conditioned for not less than 2h, using direct voltage. The oil and its solutions treated in this way were then tested with uniform and nonuniform electrode configurations.With the electric strength of the treated oil as a reference, the effect on its value of different additives at different concentrations was examined. It was found that the presence of all the additives which were used affected the breakdown strength of the oil. Optimum concentrations of additives were observed, which gave maximum increases in breakdown values.The paper also gives the results of electric-breakdown tests on liquid paraffin and its solutions, containing aromatic constituents of transformer oil as well as hydrogen acceptors. The test samples of liquid paraffin were treated in a similar way to those of transformer oil and tested in the same apparatus. Again using the electric strength of pure liquid paraffin as a reference, the effects of additives were examined. The hydrogen acceptors, when present in liquid paraffin, were found to be less effective than in transformer oil. More pronounced effects of these additives were found, however, when the solution of liquid paraffin also contained a small amount of aromatic constituents of transformer oil.A breakdown model is suggested for both liquids, based on the assumption of two types of gas bubbles which are formed when the liquids are subjected to high electric stresses. This model gives a reasonable explanation of the experimental observations.