The changes of Mn–Y (Y=S, Se, Te) bond lengths R with the Mn composition x in the diluted magnetic semiconductors Zn1−xMnxY are calculated from the optical spectra and their pressure shifts. The calculated results suggest that the Mn–Y bond lengths change little with the Mn composition x—much less than expected from lattice parameters as determined by X-ray diffraction. These results are also in good agreement with the observed values obtained from the EXAFS measurements. It appears that for ternary A1−xMxY (M denotes the divalent transition-metal ion) alloys the changes of M–Y bond lengths R with the M composition x can be studied from their optical spectrum data.