A new copper(II) compound, [Cu(L-proline)(2)](2). 5H(2)O (C20Cu2H42N4O13) (called compound I) was synthesized and crystallized, and its structure was solved using X-ray methods. It is monoclinic, space group P2(1), with a = 11.187(1) Angstrom, b = 12.172(3) Angstrom, c = 11.661(1) Angstrom, beta = 114.96(1)degrees and Z = 2. There are two chemically different copper molecules (labeled A and B), both with the copper atom In a N2O2 square planar coordination. Molecule type A has one water molecule in an apical position. Molecule B has water molecules in each of the two apical positions. Single-crystal EPR measurements have been performed in I and also in Cu(D,L-proline)(2). 2H(2)O (compound II). From the similar angular variations of the position of the single resonance observed in both compounds, we evaluated the molecular g tensors. Interpretation of the molecular g tensors resulted in dx(2)-y(2) orbital ground states. From the angular variations of the line width we calculated the magnitude of the exchange interactions coupling neighbor copper ions in each compound. In I copper ions type A at 7.25 Angstrom are arranged in chains coupled through axial-equatorial bonds. The exchange coupling within these chains is \J/k\ = 118 mK, The coupling between copper ions type B is weaker. However, the interactions between copper ions type A and B generate a three-dimensional magnetic network. Our data in compound II indicate that a superexchange pathway containing a weak hydrogen bond C-H- - -O is the path for an exchange interaction with \J'/k\ = 48 mK between coppers in neighbor layers at 9.75 Angstrom.