A model is presented to predict the flow split of horizontal air-water annular flow at a T-junction with a side arm at an arbitrary inclination to the horizontal. The distribution of the film thickness and him flow rate around the circumference of the inlet pipe is predicted and the results implemented in a model to determine the flow split at a T-junction. Under certain conditions, an abrupt increase in the amount of liquid extracted into the side arm is observed due to liquid coming to rest in the main pipe. A method is presented to include this phenomenon taking into account the distribution of film flow rate. Predictions are shown to be in good agreement with available flow split data taken at normal and reduced T-junctions with main pipe diameters of 0.032 and 0.038 m with horizontal, vertically upward and vertically downward side arms.