The effect of ethanol on the thermodynamic properties on two anionic amphiphilic penicillins, cloxacillin and dicloxacillin, has been investigated. Cloxacillin and dicloxacillin are two molecules that are similar structurally, differing only by an additional chlorine atom on the phenyl ring of dicloxacillin. The penicillins can be considered as hydrotropes if we considered that the term comprises hydrophilic and hydrophobic moieties that form aggregates by a stacking mechanism as is the case of both penicillins. By means of ultrasound velocities and densities, we have calculated the apparent molar volumes and adiabatic compressibilities. The critical concentrations, cc, and partition coefficients, K, have been determined, the latter using an indirect method based on the pseudophase model with the help of apparent molar data. This method has the advantage of allowing one to calculate the distribution coefficients at concentrations of the solubilizate below the saturation. The standard molar energy of Gibbs change, DeltaG0, on transfer from the aqueous to the micelar phase was calculated from the partition coefficient. The effect of the alcohol involves a slight decrease of the critical concentration because of a headgroup repulsion decrease. The enthalpies of dilution of dicloxacillin in a mixture of water and 15% w/v of ethanol were calculated. The aggregation process is more exothermic in ethanol that in pure water.