Batch bioreactor cultivations using Saccharomyces cerevisiae at high (190-305 g l -1 glucose) or low (21-25 g l -1 glucose) gravity conditions were monitored on-line using multi-wavelength flourescence (MWF) and standard monitoring sensors. Partial least squares models were calibrated for the prediciton of cell dry weight (CDW), ethanol and consumed glucose, using the two data types separately. The low gravity cultivations (LGCs) consisted of two phases (glucose consumption with concomitant ethanol production followed by ethanol consumption after glucose depletion), which proved difficult to model using one and the same model for both phases. Segmented modelling, using different models for the two phases, improved the predictions significantly. The prediction models calibrated on standard on-line process data displayed similar or lower root mean square error of prediction (RMSEP) compared to the flourescence models. The best prediction models for high gravity cultivations (HGCs) had RMSEPs of 1.0 g l -1 CDW, 1.8 g l -1 ethanol and 5.0 g l -1 consumed glucose, corresponding to 4%, 2% and 2% of the respective concentration intervals. Corresponding numbers in low gravity models were 0.3 g l -1 CDW, 0.7 g l -1 ethanol and 1.0 g l -1 consumed glucose, corresponding to 4%, 8% and 4% of the respective concentration intervals.