A series of carbonylbis(dimethylglyoximato)(substituted pyridine)iron(II) complexes, [Fe(Hdmg)₂(X) CO](I; X = substituted pyridine), has been prepared and analysed, and visible, near u.v., i.r., and Mössbauer spectra have been measured in the solid state. Two charge-transfer (c.t.) bands of the reflectance spectra are assigned to electronic transitions from the metal atom to molecular orbitals of Hdmg and CO, respectively, on the basis of their dependence on pK values of X. Of the two Mössbauer parameters, the isomer shift, δ, and the quadrupole splitting, ΔE_Q, the first appears to be linearly correlated with pK values of the ligand X, while the second correlates linearly with the CO stretching frequency. It appears that delocalization of d_xz electrons on the CO group dominates the ΔE_Q values, through the mediating influence of the σ-donor power of X (in one series) and the π-acceptor power of the bases (for the remaining complexes).