The imidazole of chromophoric p-(dimethylamino)benzoic acid, DABIm, reacts with the serine protease alpha-chymotrypsin in the pH range of 4-7 to form a stable acyl intermediate that gives very good resonance-enhanced Raman spectra. The resonance Raman and absorption spectra of the acyl enzyme intermediate have been compared with the spectra of simple model compounds such as the corresponding chromophoric methyl ester, aldehyde, and imidazole. The resonant Raman and ultraviolet absorption spectra of these simple chromophoric model compounds change considerably with the solvent. However, each of the model compounds exhibits a linear correlation between the maximum wavelength of absorption and the frequency of the carbonyl vibration. The observed values of the acyl intermediate do not fall on the line for the methyl ester but rather on the line for the aldehyde. This shows that the chromophoric serine ester of the acyl enzyme behaves differently than an ordinary ester, which cannot be explained as a solvent effect. Thermal unfolding of the acyl enzyme brings the spectroscopic parameters close to those of the model ester. We conclude that it is the specific conformation of the native enzyme and not solvent effects that change the spectroscopic properties of the acyl chromophore. It is reasonable that these changes arise from the same forces that cause the catalytic events. The carbonyl frequencies of a series of para-substituted benzoyl methyl esters show a remarkably linear correlation with the rate of deacylation of the corresponding acyl enzymes.