The load-carrying capacity, friction reduction, and antiwear properties of two 1,3,4-thiadiazole derivatives added to liquid paraffin under boundary lubrication conditions were evaluated using a four-ball test machine. Their anticorrosive and antioxidative properties and thermal stability were also examined. The results when compared with those of zinc butyloctyldithiophosphate (ZDDP) show that the two novel compounds possess an excellent load-carrying capacity and better antiwear and antioxidative properties than ZDDP. In addition, they have good anticorrosive properties and high thermal stability. The rubbed surface was investigated using X-ray photoelectron spectroscopy and electron probe microanalysis. The extreme-pressure antiwear and antioxidation mechanisms are discussed. (C) 2000 Elsevier Science Ltd.