Thermal decomposition temperatures of oil blends of the sine, nickel, and cadmium salts of the same dialkyldithiophosphoric acid were determined by the use of the isoteniscope. It was observed that the ease of decomposition and the order of stability of these metal dialkyldilhiophosphales were affected by: (a) the nature of the base oil used for the blends, (b) the presence of basic barium sulfonate in the blends, (c) the presence of air in the isoteniscope, and (d) the pressure in the isoteniscope.