The culture of Chinese hamster ovary (CHO) cells to produce monoclonal antibodies (MAb) requires accurate measurement and control of pH. Unwanted pH drifts in cell culture can adversely affect process performance, product quality, and product yield. To measure and control pH throughout the length of a culture, most cell culture processes use traditional glass pH probes. Several variables can affect the design and performance of glass pH electrodes and lead to drift in the measurement. Understanding these variables and their effects on pH performance can lead to design improvements and potentially reduce the drift. In this study, a set of Rosemount Analytical glass pH probes was investigated in cell culture operations. Electrochemical properties of the probes were monitored throughout the experiments. Experimental results show that the glass membrane potential experiences the biggest change during cell culture operations. Changes in the reference electrode potential are small compared with the changes in glass membrane potential. The glass membranes are affected by the steam sterilization process and this is the main cause for drift in the probe sensing signal during cell culture operations. Steam sterilization can cause the potential of glass membranes to change by up to 15 mV (similar to 0.25 pH units). This change in membrane potential can be observed as an undesirable pH drift in bioreactors. (C) 2011 American Institute of Chemical Engineers Biotechnol. Prog., 27: 885-890, 2011