The system formed by Cu(II) and 2,2'-bipyridine (bpy) has been revisited by use of EPR and electronic absorption spectroscopies. The mono-chelated [Cu(bpy)] 2+ , bis-chelated trans-[Cu(bpy) 2 ] 2+ with pseudo-tetrahedral geometry due to the steric hindrance of the 3,3' hydrogen atoms and tris-chelated [Cu(bpy) 3 ] 2+ species exist in aqueous solution. In basic media hydrolysis yields [Cu(bpy)OH] + and [Cu(bpy)(OH) 2 ]. The dimer [Cu(bpy)(OH)] 2 2+ is favoured by increasing metal ion concentration in the pH range 8-10. The EPR spectrum of the dimeric species shows well-resolved ΔM=1 and 2 features, and is different from that observed in the solid state. The zero-field splitting is consistent with a distance of ~3.4 Å between metallic centres. The behaviour of the complex system is in agreement with potentiometric data reported in the literature. In the EtOH-H 2 O mixture the bis-chelated complex adopts a five-coordinate cis structure exhibiting a rhombic EPR spectrum and, in basic solution, the [Cu(bpy) 2 (OH)] + species is formed. The attribution of the solution species has been confirmed by the analysis of solid compounds, [Cu(bpy)SO 4 ]•2H 2 O, [Cu(bpy)(H 2 O) 2 ](NO 3 ) 2 , [Cu(bpy) 2 OSO 3 •4H 2 O and [Cu(bpy) 2 ONO 2 ]NO 3 •H 2 O, after dissolution.