The transitions from trickle to bubbly flow and trickle to pulsing flow are investigated in the range of gas superficial velocities v(G) = 4-220 mm/s using air and water, with additional consideration of the effect of the prewetting procedure. The flow regime transition was detected by the standard deviation of the pressure drop and visual observation, with further confirmation using a high-speed camera. Results show a significant effect of the prewetting procedure on the liquid superficial velocity at a fixed gas superficial velocity required for transition from trickle to bubbly flow and trickle to pulsing flow. At low gas superficial velocities, where the transition from trickle to bubbly flow occurs, a significant departure from literature model predictions is observed. Below VG = 20 mm/s, rather than the expected increase in the liquid superficial velocity required for transition with decreasing gas superficial velocity, the transition is observed to be essentially independent of the gas flow.