A high pressure apparatus based on two sapphire windows high pressure cell coupled with a detection system of the reflected and refracted light intensities coming from a laser beam was designed to determine the phase transitions by the measurement of the light intensity change. The setup was used to measure the wax disappearance temperature under pressure up to 100 MPa in pure component, synthetic complex mixtures made up of distributions of n-paraffins ranging from n-C-20 to n-C-42. Finally a real stabilized condensate was successfully investigated and the sensitivity of the detection system was demonstrated on a dark crude oil. (c) 2007 Elsevier Ltd. All rights reserved.