New complexes of lanthanum(III), cerium(III) and europium(III) with para- and meta-substituted benzeneseleninic acids of the type XC6H4SeO2H (X = H, m-Cl, p-Br, p-Me) are reported. The compounds Ln(XC6H4SeO2)3, Ln(XC6H4SeO2)2Cl, Ln(XC6H4SeO2)Cl2 and Ln2(XC6H4SeO2)3Cl3, where Ln denotes trivalent La, Ce, and Eu, have been studied using spectroscopic techniques (ir, far-ir and electronic spectra), magnetic susceptibility, thermogravimetric and conductivity measurements. The electronic spectra of the 1 : 3 derivatives have been analysed and spectral parameters are calculated and discussed for the europium complexes. The ir data point to a seleninato-O,O' coordination for all the complexes; in particular, the presence of three SeO bands with irreducible representation A 2 + 2E in the ir spectra of the tris derivatives suggests an octahedral configuration with D 3 symmetry. All the halo derivatives are polymeric and octahedral in nature with bridging halide atoms. The magnetic moment values lie in the expected ranges and show very little deviation from Van Vleck values.