The basicity of pyrazine bound to [M-II(CN)(5)](3-) centers (M = Fe, Ru, Os) was studied by spectrophotometric titrations in the pH range 0-7. The overlapping spectral picture is associated with competitive and successive protonation steps. Multiwavelength absorption data were treated by factor-analysis procedures to disclose the pK(a) for the [M(CN(5)pzH](2-) species (1.24; 0.41; 0.53 for M = Fe, Ru, Os, respectively). The order of increasing basicity, Fe > Os > Ru, was consistent with the order in the chemical shifts for the beta-protons in the pyrazine ring: 8.24 (Fe) < 8.36 (Os) < 8.45 (Ru). pK(a) values were also obtained for the corresponding [(HNC)M(CN)(4)pz](2-) ions.