pH measurements in blood or in media containing either proteins or polypeptides and performed by means of micro pH combination electrodes type N 58 (Schott & Gen., Mainz) yield in a systematic error according to the regression line y = 1.135 chi - 0.842, in the range between pH 5.3 and 8.3. This deviation from the real pH value is independent of the protein concentration and amounts to 0.1-0.2 pH units in the physiological range. The error does not occur if the pH measurements are performed in media which are free of proteins and polypeptides, respectively. If the electrolyte solution within the reference electrode is replaced (NaCl solution instead of KCl solution) the error is distinctly reduced. For this reason, this deviation should be caused by the variation of the diffusion potential across the platinum junctions.