Two crystalline compounds of rare-earth chlorides coordinated with L-proline, [RE(L-Pro)(2)(H2O)(5)]Cl-3 (RE = Ho, Er), were synthesized. Their structures were characterized using thermogravimetric analysis (TG), differential thermal analysis (DTA), and chemical analytical methods, the purities of the [RE(L-Pro)(2)(H2O)(5)]Cl-3 (RE = Ho, Er) complexes were determined to be 98.85 % and 98.62 %, respectively. After the measurement of the dissolving heat of the two systems, [RE(NO3)(3)center dot 6H(2)O + 2L-Pro + 3NaCL] and {[RE(L-Pro)(2)(H2O)(5)]Cl-3 3NaNO(3)}, at 298.15 K in 2 mol . dm(-3) HCl solvent by using a conduction microcalorimeter, the standard enthalpies of formation of the two crystalline compounds at 298.15 K were determined to be Delta H-f(m) (circle minus){[Ho(L-Pro)(2)(H2O)s]Cl-3, s, 298.15 K} = (-3675.0 +/- 1.3) kJ . mol(-1) and Delta H-f(m) (circle minus){[Er(L-Pro)(2)(H2O)(5)]Cl-3, s, 298.15 K} = (-3655.9 +/- 1.6) kJ . mol(-1), respectively.