Powder and single-crystal optical absorption of the ferrimagnet (MnCuII)-Cu-II(pba)(H2O)(3). 2H(2)O (denoted MnCu) and the Mn-doped compound Mn0.1Mg0.9Cu(pba)(H2O)(3). 2H(2)O (denoted Mn0.1Mg0.9Cu) with pba standing for 1,3-propylenebis(oxamato) was investigated in the 10-300 K range. The crystal structure of MnCu was previously reported, and consists of bimetallic chains with octahedral Mn-II and square pyramidal Cu-II ions linked by oxamato bridges, MnCu and Mn0.1Mg0.9Cu being isostructural. The spectra of both MnCu and Mn0.1Mg0.9Cu show an important dichroism along the chain direction, due to the strong polarization of the Cu-II band at around 16 000 cm(-1) in this direction. They exhibit narrow and intense spin-forbidden Mn-II transitions in the 24000-25000 cm(-1) range, which are activated by an exchange mechanism. The polarization and thermal dependence of the (6)A(1g) --> (4)A(1g), E-4(g)(G) Mn-II transitions were recorded. The polarization along the chain axis was interpreted in the framework of the pair mechanism first introduced by Tanabe and co-workers. A theoretical expression for the thermal dependence of the intensity was derived by considering the Cu-II spin as a quantum spin and the Mn-II spin as a classical spin, and compared with the experimental data. The interaction parameter between the local ground states has been found to be J = -25 cm(-1) using the spin Hamiltonian H = -J Sigma(i)(S(Mn,i)s(Cu,i) + S(Mn,i+1)s(Cu,i)). The spectra of Mn0.1Mg0.9Cu showed cold and hot bands, whose energy difference is directly related to J and the interaction parameter J* between the Cu-II ion in its ground state and the Mn-II ion in its spin-flip excited state. J* has been estimated to be +40 cm(-1). These results have been compared to those obtained with other (MnCuII)-Cu-II compounds. The complementarity between optical and magnetic properties has been discussed.