In this paper an experimental investigation of the heat transfer in mini channels with a hydraulic diameter of 889 m is conducted. The method used is planar laser induceduorescence (PLIF), which uses the principle of laser excitation of rhodamine B in water. The goal of this study is to validate the applicability of PLIF to determine the convective heat transfer coecient in mini channels against conventional correlations of the convective heat transfer coecient. The applicability of the conventional theory in micro and mini channels has been discussed by several researchers, but to the authors knowledge the applicability of PLIF to validate this has not yet been investigated thoroughly. The experiment shows good agreement to the conventional correlation, and the resolution of the temperature gradient at the wall is found suciently accurate in certain areas. However, PLIF is not found satisfactory over the whole domain, and the limitations and errors are analysed.