The solubility in mole fraction Xs of tris(2,4-pentanedionato)cobalt(III) (Co(acac)3) in aqueous mixtures of 2-butoxyethanol (BE) and 2-ethoxyethanol (EE) has been determined over the temperature range 5–50 °C. From these data, the enthalpy [Formula: see text] and heat capacity of solution [Formula: see text] of Co(acac)3 are estimated by using the Clarke and Glew procedure. For the water–BE system, an approximately linear correlation is found between the excess enthalpy of solution [Formula: see text] of Co(acac)3 and the excess internal pressure δPi of the mixtures at 25 °C. This finding suggests that the composition dependence of [Formula: see text] in this system is mostly governed by the composition dependence of Pi. On the other hand, [Formula: see text] in these mixtures exhibits a characteristic composition dependence and its composition dependence in the water–BE mixtures may be qualitatively explained by considering an effect of the microphase transition which has been shown to occur in the mixture. The composition dependence of both [Formula: see text] and [Formula: see text] in the water–BE mixture is analogous to that in the water–BE mixture and this finding suggests that a microphase formation may also occur in the former mixture. Keywords: tris(2,4-pentanedionato)cobalt(III), temperature dependence of solubility, enthalpy and heat capacity of solution, aqueous mixtures of 2-butoxyethanol and 2-ethoxyethanol, microphase transition.