The monitoring of heat exchangers on duty with respect to flow mal-distributions needs the development of inlet-outlet experimental techniques in order to perform a fault diagnosis. Within this framework and as a first attempt to solve this problem in the case of multi-phase heat exchangers, we propose to use a non-intrusive optical sensor associated to a liquid phase tracer experiment. In order to check the capabilities of this technique, we present here the results that we have obtained for the characterization of two phase flows in horizontal and vertical tubes. The difference between refractive index of the two phases allows estimating the void fraction on the section illuminated by the optical sensor and permits to characterize two-phase flow regime from the signal characteristics. Signal analysis and treatment of the absorbance variation due to the coloured tracer injected in the liquid phase permit to estimate the real liquid phase averaged velocity and consequently the real gas phase average velocity. We also calculated volumetric void fraction and compared it to usual correlations with a good agreement. Finally, the evolution of the experimental liquid phase Peclet number with the two-phase flow regime has been observed and qualitatively explained. (C) 2007 Elsevier Ltd. All rights reserved.