The influence of the macrocycle core size on the keto and acetyl carbonyl stretching wavenumbers of bacteriochlorophyll a and of its Co, Ni, Cu and Zn derivatives was examined. Their sensitivity to the polarity and polarizability of the molecular environment was also studied. It is concluded that the macrocycle core size has a clear, but limited, influence on these wavenumbers. As a consequence, interchange between the five- and six-coordinated states of the central Mg atom of bacteriochlorophyll a should shift the keto and acetyl carbonyl stretching wavenumbers, but by not more than 3 and 1 cm(-1), respectively. In line with previous observations on chlorophylls, a marked dependence of the keto and acetyl carbonyl stretching wavenumbers of bacteriophaeophytin a and of bacteriochlorophyll a and its Ni and Zn derivatives on solvent permittivity and refractive index was observed. The sensitivity of the keto carbonyl wavenumber appears to be largely independent of the presence and nature of the central metal and is very close to that reported earlier for chlorophyll a. The wavenumber of the acetyl carbonyl stretching mode of bacteriochlorophyll a derivatives is overall less sensitive to any of these parameters than that of the keto carbonyl group.