Solid phase microextraction (SPME) coupled with GC has been applied to study the binding properties between bovine serum albumin (BSA) and volatile organic compounds such as benzene, toluene, ethylbenzene, propylbenzene and butylbenzene. Their protein-ligand equilibrium constants have been determined. The measurement of free and bound ligand concentrations in the aqueous solution was based on the equilibrium among the analyte in the fiber coating (C(f)), headspace (C(h)) and aqueous solution (C(s)). The work demonstrated that SPME is a simple and effective method in the study of protein binding to measure the freely dissolved analyte concentration as well as the equilibrium constant. The theoretical aspect of the SPME applied to the equilibrium constant measurement in two-phase (liquid sample-fiber coating) and three-phase (liquid sample-headspace-fiber coating) systems has been thoroughly discussed. The results demonstrated that the interpretation of the calibration data is crucial to the determination of freely dissolved analyte concentration and the equilibrium constant especially when the sample volume is small. The error in the experimental system is discussed. It is demonstrated in this study that for the three-phase system the amount of the analyte partitioned in the headspace could be ignored only in certain circumstances, where the Henry's law constant and the ratio between headspace volume and sample volume are sufficiently small.