A calorimetric study of the RNi2 Laves phases (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho and Er) has been carried out to determine the heat of formation (Delta H-f degrees) at 300 K using a high temperature direct drop calorimeter. The reliability of the calorimetric results has been determined and supported by using different analytical techniques: light optical microscopy, scanning electron microscopy equipped with electron probe microanalysis (EPMA) with energy dispersive X-ray spectroscopy (EDS) detector and X-ray powder diffraction analysis. We observed a gradual regular decreasing trend of the heat of formation with the increasing of the rare-earth atomic number from -34 +/- 1.5 kJ/mol for PrNi2 to -50 +/- 1 kJ/mol for ErNi2. Results have been discussed and compared with the data previously reported in the literature. (c) 2013 Elsevier Ltd. All rights reserved.