This work provides an in-situ method for determining the flow regime in a lab scale gas-liquid stirred tank reactor based on optical probe measurements. Tapered (conical) end optical fibers, which can distinguish which phase their tips are surrounded by, were employed over the whole range of practical operating conditions achievable in our Chemical Reaction Engineering Laboratory (CREL). After checking for sources of error associated with the rise and fall times of the measured signals, gas holdup and bubble count profiles were obtained by processing the time-series data with appropriate in-house developed algorithms. The data were presented in terms of the two dimensionless numbers, the Flow Number (FI) and the Froude Number (Fr). All experiments were executed with an air-water system but the technique can be employed with all liquids and gases. The results suggest that the optical probe, when strategically positioned, can successfully and readily determine which state of dispersion the reactor is in. This reveals the technique's potential usefulness as an important research and control tool. (c) 2014 Elsevier Ltd. All rights reserved.