The standard molar enthalpies of solution of Dy metal and monoclinic DyCl3(s) in 4.0 mol . dm(-3) HCl(aq), and of NdCl3(s) in 2.3 mol . dm(-3) and 4.0 mol . dm(-3) HCl(aq) have been measured. From these values, the standard molar enthalpies of formation have been derived : Delta(f)H(m) degrees(DyCl3, s, T = 298.15 K) = -(994.3 +/- 1.3) kJ . mol(-1) and Delta(f)H(m) degrees(NdCl3, s, T = 298.15 K) = -(1040.1 +/- 0.8) kJ . mol(-1). The results are compared with previous values from the literature.