An on-line pH monitoring method based on mid-infrared spectroscopy relevant to bioprocesses is presented. This approach is non-invasive and does not require the addition of indicators or dyes, since it relies on the analysis of species of common buffers used in culture media, such as phosphate buffer. Starting with titrations of phosphoric and acetic acid solutions over almost the entire pH range (2–12), it was shown that the infrared spectra of all samples can be expressed as a linear combination of the molar absorbance of the acids and their deprotonated forms. In other words, pH had no direct influence on the molar infrared spectra themselves, but only on deprotonation equilibria. Accurate prediction (standard error of prediction for pH < 0.15 pH units) was achieved by taking into account the non-ideal behavior of the solutions, using the Debye–Hückel theory to estimate the activity coefficients. Batch cultures of E. coli were chosen as a case study to show how this approach can be applied to bioprocess monitoring. The discrepancy between the spectroscopic prediction and the conventional electrochemical probe never exceeded 0.12 pH units, and the technique was fast enough to implement a feedback controller to maintain the pH constant during cultivation. Biotechnol. Bioeng. 2008;100: 82–93. © 2007 Wiley Periodicals, Inc.