The electronic properties of cobalt(II)- and nickel(II)-substituted azurins from Pseudomonas aeruginosa have been investigated. EPR data for the cobalt derivative and paramagnetic susceptibility data for the nickel derivative are reported. The EPR spectrum of Co(II)-azurin shows the typical pattern of a Kramers’ doublet (+/- 1/2) associated with an S = 3/2 ground state in a distorted axial symmetry environment. The temperature dependence of the EPR intensities shows that this Kramers’ doublet is the excited doublet and, therefore, that the corresponding zero-field splitting parameter D is negative (similar to -3.5 cm(-1)). The mean g value is equal to 2.3. Nickel(II) azurin exhibits an effective magnetic moment mu(eff) = 2.8 mu(B) (Bohr magnetons), constant in the temperature range 120-30 K. The magnetic moment decreases and reaches the value of 1.80 mu(B) at 5 K. From the temperature dependence of the susceptibility, the fitting of the data to the theoretical S = 1 susceptibility equation leads to a zero-field splitting parameter D of around 17.7 cm(-1). The spin Hamiltonian parameters that have been determined for the two metallosubstituted proteins are consistent with a highly distorted tetrahedral structure derived from an axially elongated trigonal bipyramid.