A novel type of photo-bioreactor is being studied. The main feature in this system is the inclined tube in which most of the photosynthesis occurs. Two phase gas-liquid counter-current flow takes place in this environment. The flow configuration in the range of interest has not been described previously. The trajectories of the liquid phase are the target of the present work. These trajectories are needed for a proper mathematical representation of the process. Experimental studies where carried out using two methods: studying the response of the system to a pulse disturbance, and tracking an optical tracer with a system developed in our laboratory, The influence of tube inclination and of gas flow rates was studied. A rough compartmental model is presented, which fits the transient experimental data. The mass transfer rate from the gas phase to the liquid was studied since it is needed to assess the capacity of providing enough CO2 to match the light captured by photosynthesis. Both gas holdup and mass transfer rates are reported as a function of tube inclination and gas flow rates. (C) 2007 Published by Elsevier Ltd.