Reaction of Cu(ClO4)2 with 4,6-dimethyl-2-hydroxypyrimidine (Hdmpymo) in aqueous amine solution gives rise to compounds of formula [L2Cu(dmpymo)](ClO4) (L=NH3, 1a; MeNH2, 1b). An X-ray crystallographic study on 1a revealed a 1-D polymeric structure in which dmpymo entities symmetrically bridge square planar copper(II) centres. The copper environment is made up of two ammonia and two nitrogen donor atoms from the pyrimidine ligands disposed in a trans configuration. Spectroscopic data (electronic spectra, EPR, IR) for 1a and 1b are very similar revealing their related nature. The magnetic properties of 1a have been investigated in the temperature range 2–300 K. A significant antiferromagnetic coupling with J=−23.0 cm−1, through the pyrimidine bridges, is observed by analysing the susceptibility data with the Hamiltonian H=−2JSiSi+1. Crystal data: orthorhombic, space group Pnma, a=13.676(3), b=10.804(2), c=9.528(2) AD , V=1407.8(5) AD 3, Z=4, Dcalc =1.681 g cm− 3, R=0.037 for 999 reflections.