Abstract: The fast atom bombardment (FAB) mass spectra of free bile acids and their succinyl derivatives are described. Using glycerol or thioglycol as "matrix compatible solvent" of the samples studied, it is noted that various matrix is different in enhanced sensitivity. In FABnegative ion mass spectra the typical (M- n)^- cluster ion is observed and the (M- 1)^- ion shown as its highest intensity is the base peak for each of the samples.