Optical band gap (E o p t ) for Pb modified Ge-Se-Te bulk glasses has been measured using the photoacoustic (PA) technique. The advantage of this technique over the absorption spectrophotometry technique is that this method is independent of sample thickness and provides optical band gap energy (E o p t ) values corresponding to a high (~10 4 cm - 1 ) absorption coefficient. Hence this technique is especially suited for highly absorbing and brittle bulk semiconductors. The composition dependence of the optical energy E o p t of two series of glasses, namely, Pb x Ge 4 2 - x Se 4 8 Te 1 0 (3=<x=<13) and Pb 2 0 Ge x Se 7 0 Te 1 0 (17=<x=<24) has been measured by recording the variation of the PA signal as a function of wavelength in the range 400-1200nm. E o p t has a minimum value for the composition with 9at.wt.% Pb in the Pb x Ge 4 2 - x Se 4 8 Te 1 0 series of glasses. E o p t exhibits a maximum for the 21at.wt.% Ge composition in the Pb 2 0 Ge x Se 7 0 Te 1 0 series of glasses. The above results have been interpreted in terms of the variation of the average bond energy of these glasses with change in composition. It is a matter of interest that E o p t shows anomalous behaviour at compositions at which the majority charge carrier reversal is observed in these glasses.