The composition of oil trapped in fluid inclusions cannot be accurately determined with conventional techniques. Consequently, pressure and temperature trapping conditions in petroleum environments will only be established by a combination of volumetric and homogenisation-temperature data. Therefore, analytical procedures and limitations of Confocal Scanning Laser Microscopy (CSLM) have been determined for the volumetric approximation of oil inclusions. The volumetric analysis of an oil inclusion has been obtained using a confocal BIO-RAD inverted scanning laser microscope. 68 focus planes spaced at 1 mu m have been acquired and computerised. Fluorescence image processing (thresholding) allows the oil volume to be estimated. Contouring of each focus plane and surface modelling by GOCAD software to produce a three-dimensional model of the inclusion revealed optical aberration at the level of the vapour bubble. Consequently, the vapour bubble may be approximated as a sphere and a gas vol.% can be calculated from different estimate procedures (voxel counting, surface modelling). The result has shown that the average n-alkane composition corresponding to the estimated gas vol.% of the inclusion is close to the average n-alkane composition deduced by FT-IR analysis. This work has demonstrated the ability of CSLM to estimate the volume of the cavity of an oil inclusion to an accuracy better than 95%.