The infrared spectrum of 1:1 complex of mercury(II) saccharinate with 2,2′-bipyridine, [Hg(C7H4NO3S)2(C10H8N2)], was studied in the CO and SO2stretching regions. The appearance of the spectrum in the region of the carbonyl stretching modes was correlated with the number of non-equivalent CO groups in the structure. The spectral and structural characteristics of the CO groups in the title compound were compared with the corresponding ones in the mercury(II) saccharinate and chloromercury(II) saccharinate. It was found that the frequency of the CO stretchings in the spectrum of Hg(bpy) (sac)2is significantly lower (1630 and 1615 cm−1) than the frequency of the corresponding modes in the spectra of covalently bonded mercury(II) saccharinate (1705 and 1680 cm−1) and chloromercury saccharinate (1694 cm−1). An attempt was also made to assign the bands which are mainly due to the symmetric and the antisymmetric SO2stretching vibrations.