Using especially designed and constructed isothermal calorimeter, the austempering of Cu-Ni ductile iron was carried out at temperatures of T-pi =270,350,390 and 430 degrees C, simultaneously with the measurement of accompanying thermal effects. The values of enthalpy changes Delta H show the correct and expected trend, varying from 45 J/g at T-pi =430 degrees C to 70 J/g at T-pi = 270 degrees C. The changes of enthalpy were observed to increase with decreasing temperature of the isothermal transformation during austempering. The results of the present work did not allow revealing the influence of alloying elements, i.e. Cu and Ni, on the heat of isothermal transformation. The discrepancies between the results of the actual measurements and the results quoted in other papers were discussed in terms of DSC measurements. (C) 2009 Elsevier B.V. All rights reserved.