An optical-electronic system was developed for the determination of film thicknesses in falling film reactors and applied to the analysis of methyl esters sulphonation. Calibration and experimental tests were performed to obtain a correlation between the film thickness (d ) and the measured Voltage (V) of the photo electronic cell. Voltage sensing and film thickness calculations were found to be highly dependent on the position of the sensing device from the reactor inlet and slightly dependant on the gas flow rate under the range of experimental values tested. This optical-electronic system allows a real-time quantitative description of the dynamic behavior of the undulations in the falling film.