Iridium(I) (acetylacetonato)(1,5-cyclooctadiene) complex, Ir(C5H7O2)(C8H12), was synthesized and characterized by elemental CH-analysis, NMR, X-ray spectroscopy. The heat capacity of compound Ir (C5H7O2)(C8H12) has been measured by the adiabatic method within the temperature range (6-318) K. The thermodynamic functions: entropy, enthalpy and reduced Gibbs energy have been calculated. They have the following values at 298.15 K: Cp degrees = (323.6 +/- 0.6) J K (1) mol (1), Delta S-298.15(0)m degrees = (347.0 +/- 0.9) J K (1) mol (1), Delta H-298.15(0)m degrees = (50.82 +/- 0.09) kJ mol (1), Phi(m)degrees = (176.5 +/- 0.7) J K (1) mol (1). The value of the absolute entropy were used to calculate the entropy of formation of Ir(C5H7O2)(C8H12) at T = 298.15 K. An anomaly of heat capacity has been discovered with a maximum at T-c=287.20 K, which points to the phase transition of the complex. Anomalous contributions to entropy and enthalpy have been revealed. (C) 2016 Elsevier Ltd. All rights reserved.