Experimental information and theoretical estimates for the enthalpy of formation at 298 K for the RM2 (whereM = Fe, Co,orNi andR = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu) lanthanide Laves phases with the MgCu2 (C15) structure are reviewed and discussed in the light of information on lattice parameters and atomic volume. The purpose of the present study is to analyze all the available experimental information on enthalpies of formation for transition-metal lanthanide Laves phases and then plot the data in order to reveal tendencies and allow reliable estimates of data that have not yet been measured. It is concluded that as the lanthanide atomic number increases the enthalpy of formation is almost constant or becomes slightly more negative. The accompanying volume contraction is slightly larger than expected from the lanthanide contraction, 7.6 %. From the so-called Gschneidner correlation one would have expected the contrary behavior, that is, that the bonding strength would become slightly lower when going from LaM2 to LuM2. An increase in the transition-metal atomic number has a stronger effect on the enthalpy of formation and atomic volume than an increase in the lanthanide atomic number.