The present report studies on the flow pattern transitions during vertical air water downflow through millichannels (0.83Eotvos no.20.63). Four basic flow patterns namely falling film flow, slug flow, bubbly flow, and annular flow are observed in the range of experimental conditions studied and their range of existence has been noted to vary with tube diameter and phase velocities. Based on experimental observations, phenomenological models are proposed to predict the transition boundaries between adjacent patterns. These have been validated with experimental flow pattern maps from the present experiments. Thus the study formalizes procedure for developing a generalized flow pattern map for gas-liquid downflow in narrow tubes. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 792-800, 2017