New experimental data for air-water flow in a horizontal square cross-section channel (H=24.25 mm) is presented, including data on liquid hold-up, gas and liquid velocities, and wave velocities and frequencies. For the majority of gas and liquid flow rates studied, the regime observed was pseudo-slug. Using visualization studies it was possible to identify wavy-stratified and pseudo-slug flows. For the pseudo-slug regime new correlations were obtained for liquid hold-up, for gas and liquid velocities as a function of the ratio between gas and liquid mass flow rates, and for the frequency of roll-waves as a function of gas and liquid mass flow rates.