The design of process equipment involving minichannel structures necessitates detailed knowledge on the flow conditions inside the apparatus. Here, flow regimes in empty and sphere-packed square channels were investigated by using optical flow visualization with a high-speed camera. Flow maps as well as new correlations to predict the flow regime transitions based on the dimensionless gas and liquid Weber numbers are presented. Segmented flow regimes occurred in the studied empty and packed channels for very low gas and low liquid Weber numbers. Such a liquid flow in separate compartments may contribute to the design of highly efficient contacting apparatuses.