Routine cell culture is done in small-scale disposable vessels (typically 0.1-100mL volumes) in academia and industry. Despite their wide use in bioprocess development (i.e., process optimization and process validation), miniature process scouting devices (PSDs) are considered "black boxes" because they are generally not equipped with sensors. In this study, we show that on-line monitoring of dissolved oxygen (DO) and pH in a T-75 flask-based PSD can be achieved during cell passaging and that this information can be linked to different cellular metabolic states. In this case, on-line monitoring of DO and pH show three distinctive metabolic regions in passages 1-18, 19-28, 29-54 and in particular, the shift in the pH curve, the specific oxygen uptake rate (q(O2)), and the lactate production rate to the oxygen consumption rate yield (Y-Lac/ox) confirm the existence of these distinctive metabolic regions. These findings are particularly useful because they show that sensor equipped PSDs can help to monitor cell culture behavior after thaw, in pre-and seed culture prior to scale-up and in development/optimization studies. Such routine monitoring will help to develop more consistent cell culture techniques. Biotechnol. Bioeng. 2009; 105: 1040-1047. (C) 2009 Wiley Periodicals, Inc.