The activity coefficients and excess thermodynamic functions at infinite dilution were evaluated from gas chromatographic measurements for a number of binary diphenyl methyl phosphate-diluent systems. The interactions between the phenyl groups of DPMP and a diluent are predominant whereas the central phosphoryl group plays only a secondary role. A weak hydrogen bonded complete is formed between the hydrogen of chloroform and π-electrons of benzene rings. Hydrocarbons and water exhibit strong positive deviations from ideality which are sufficient to produce the phase separation. Solutions containing alcohols, toluene and carbon tetrachloride are characterized by moderate positive deviations, and ketones by small negative deviations. It was found that the DPMPC6H6 system is ideal, and that the undiluted DPMP is a complex, highly associated, viscous liquid.