Abstract: A method for evaluating the API gravity of a sample of underground formation includes steps of:Solvating a known volume of an underground formation sample in a known volume of a solvent which will solvate hydrocarbons;Quantitatively measuring with a changeable filter portable fluorometer the emission fluorescence of the solvated sample at a fixed excitation wavelength with measurements of emission intensities at two points;characterizing the oil by the ratio of two emission intensities obtained at a fixed excitation wavelength;determining the yield;applying regression analysis to a data base of oils to obtain an equation which results in an algorithm value; andinterpreting the value of the algorithm to give a value for API gravity and estimate in-situ oil concentration.