Chinese hamster ovary (CHO) bioprocesses, the dominant platform for therapeutic protein production, are increasingly used to produce complex multispecific proteins. Product quantity and quality are affected by intracellular conditions, but these are challenging to measure and often overlooked during process optimization studies. pH is known to impact quality attributes like protein aggregation across upstream and downstream processes, yet the effects of intracellular pH on cell culture performance are largely unknown. Recently, advances in protein biosensors have enabled investigations of intracellular environments with high spatiotemporal resolution. In this study, we integrated a fluorescent pH-sensitive biosensor into a bispecifc (bisAb)-producing cell line to investigate changes in endoplasmic reticulum pH (pH ER). We then investigated how changes in lactate metabolism impacted pH ER, cellular redox, and product quality in fed-batch and perfusion bioreactors. Our data show pH ER rapidly increased during exponential growth to a maximum of pH 7.7, followed by a sharp drop in stationary phase in all perfusion and fed-batch conditions. pH ER decline in stationary phase was driven by an apparent loss of cellular pH regulation that occurred despite differences in redox profiles. Protein aggregate levels correlated positively with pH ER, indicating that aggregation is not solely driven by oxidative stress and cellular protein load. Improved characterization of intracellular changes in bioprocesses can help guide media optimizations, improve bioprocess control strategies or provide new targets for cell engineering.