In this presentation we explored the quality and significance of osmolality as a predictor for titer, glucose consumption, and growth-inhibitory by-products such as ammonium in mammalian cell culture. CHO cells were cultivated in 5-liter bioreactors in fed-batch mode expressing a recombinant antibody of the subclass IgG1. In addition to the conventional on-line and at-lien measurements, osmolality was measured at-line. The measurements of the different runs were used to establish correlations with osmolality and independent runs to test the predictive power of osmolality. Interestingly the osmolality is a good predictor for glucose, ammonium, and dissolved CO2 among others with a weaker power. The implementation of osmolality measurement and its application in bioprocessing is also discussed.