Dilution enthalpies of two penicillamine enantiomers,namely, d-penicillamine, l-penicillamine, and theirracemate d,l-penicillamine, in N,N-dimethylformamide(DMF) + water mixtures of various compositions have been determinedat 298.15 K respectively, using an isothermal titration calorimeter(MicroCal ITC200). According to the McMillan–Mayer theory,homochiral enthalpic pairwise interaction coefficients (hXX) of the three penicillamines in DMF + water mixturesof different mass fractions (wDMF= 0to 0.3) have been calculated regressively. It is found that hXXcoefficients of each enantiomer and theirracemate change increasingly with wDMFto the maxima at wDMF= 0.20 and thendecrease relatively rapidly, which follows the order hLL> hDD≈ hRR> 0 (R represents the racemate mixtureofpenicillamine). The results are discussed from the point of view ofcompetitive equilibrium between hydrophobic and hydrophilic interactionsin solutions. [ABSTRACT FROM AUTHOR]