We report powder EPR measurements at 9.48 GHz and temperatures of 4 K <= T <= 300 K and at 33.86 GHz and T = 300 K for the polymeric compound {[Cu2Er2(L)(10)(H2O)(4)]center dot 3H(2)O}(n) (HL = trans-2-butenoic acid) having alternate Cu-2 and Er-2 dinuclear units bridged by carboxylates along a chain. Above 70 K, when the Er-III resonance is unobservable and uncoupled from the Cu-II ions, the spectrum arises from the excited triplet state of antiferromagnetic Cu-2 units, decreasing in intensity as T decreases, and disappearing when these units condensate into the singlet ground state. Fit of a model to the spectra at 9.48 and 33.86 GHz and 300 K gives g(Cu)(II) = 2.379, g(Cu)(perpendicular to) = 2.065, D-Cu = -0.340 cm(-1), and E-Cu similar to 0 for the g-factors and zero field splitting parameters. From the T dependence of the intensity of the spectrum above 70 K, we obtain J(Cu-Cu) = -336(11) cm(-1) for the intradinuclear exchange interaction. Below 50 K, a spectrum attributed to Er-2 units appears, narrows, and resolves as T decreases, due to the increase of the spin-lattice relaxation time T-1. The spectrum at 4 K allows calculating g values g(1) = 1.489, g(2) = 2.163, and g(3) = 5.587 and zero field splitting parameters D-Er = -0.237 cm(-1) and E-Er = 0.020 cm(-1). The results are discussed in terms of the properties of the Cu and Er ions, and the crystal structure of the compound.