Hetero-association constants and enthalpy changes are estimated for the hetero-association reactions between trifluoroacetic acid and each of p-nitro and p-meythylbenzophenones, at the temperature range 35-55 . The results show that the presence of the p-nitro group in the benzop- henone molecule weakens the hydrogen bonds formed between trifluoroacetic acid and benzop- henone, whereas the p-methyl group strengthens such bonds.