Contact angle measurements were performed for a five-ring polyphenyl ether isomeric mixture on M-50 steel in a dry nitrogen atmosphere. Two different techniques were used: (1) a tilting-plate apparatus, and (2) a sessile drop apparatus. Measurements were made for the temperature range 25 to 190°C. Surface tension was measured by a differential maximum bubble pressure technique over the range 23 to 220°C in room air. The critical surface energy of spreading (γc) was determined for the polyphenyl ether by plotting the cosine of the contact angle (θ) versus the surface tension (γLV). The straight line intercept at cos θ = 1 is defined as γc. γcwas found to be 30.1 dyn/cm for the tilting-plate technique and 31.3 dyn/cm for the sessile drop technique. These results indicate that the polyphenyl ether is inherently autophobic (i.e., it will not spread on its own surface film until its surface tension is less than γc). This phenomenon is discussed in light of the wettability and wear problems encountered with this fluid.Presented at the 40th Annual Meeting in Las Vegas, Nevada May 6–9, 1985