An experimental study is presented concerning the transition in the velocity of individual Taylor bubbles in vertical co-current liquid flow. Velocities of individual Taylor bubbles rising in co-current liquids (kinematic viscosities from 10(-6) to 5.7 x 10(-6) m(2) s(-1)) in acrylic columns of 22 mm, 32 mm and 52 mm internal diameter were measured for a wide range of Reynolds number of the flowing liquid using two nonintrusive experimental techniques. The measuring section was located at 6.0 m from the gas injection. The operating conditions used correspond to inertial controlled regime. The data showed an unexpected feature of the bubble motion: the velocity coefficient C changes even when the flow regime in the liquid ahead the bubble is still laminar, i.e., the transition in the bubble velocity starts at liquid Reynolds numbers much lower than 2100. Additional experiments, employing PIN measurements, showed a developed laminar liquid flow ahead the bubble nose. Based on a dimensional analysis, the most important dimensionless numbers for the phenomena were identified and, after processing all data, an empirical correlation was established to predict the velocity coefficient C for a large range of operation conditions. This information is very important for vertical two-phase slug flow modelling.