Weak jump anomalies were observed in the temperature derivatives of the heat capacities of aqueous 2-butoxyethanol. The derivative, (flC,/aT), is proportional to the third derivative of the Gibbs free energy with respect to T. The loci of these anomalies in the composition-temperature field fall on the boundary between two regions previously characterized by differences in the mixing scheme. Other third derivatives of free energy involving at least one composition diierentiation showed peak anomalies at the same boundary. Heat capacities, C, = -T(a2G/aT2), exhibit anomalies at bulk phase transition points. There are a great many examples of C, anomalies available in the literature, varying in shape and size. However, examples of typical second-order phase transitions in the Ehrenfest classification are rare. Rather than a J-type discontinuity, smooth peaks are much more commonly observed varying widely in their maximum values: 2.0 kB for the ferromagnetic transition of EuO at 69 K [ 11, 1.5 kB for the soft librational mode transition of P2C16 at 102 K [ 2 ],0.4 kB for the superconductivity transition of Ba2DyCu90, at 92.5 K [ 3 1, 0.1 kB for the twist transition of biphenyl at ' Carlsberg Foundation Fellow.