The optical absorption of mixed-valence sulfites such as Cu(I)(2)SO3 center dot M(II)SO3 center dot 2H(2)O [M = Cu, Fe, Mn, or Cd] were studied and the influence of the M(II) cation on this parameter. A transmission spectroscopy technique was used to measure the optical band gap energies (E-g) in the infrared region and the E-g of Cu(I)(2)SO3 center dot Cu(II)SO3 center dot 2H(2)O was estimated using a numerical fitting method that can be described empirically by E-g(chi(P)) = -0.62 chi(2)(P) + 1.88 chi(P) - 0.5, where chi(P) is Pauling's electronegativity of metal M. The obtained values were in the interval 0.85-0.95 eV, representing a narrow E-g, which can identify these compounds as potential infrared-detectors. (C) 2007 Elsevier B.V. All rights reserved.