Electrospray ionization mass spectrometry (ESI-MS) is now routinely used for the detection of cyclodextrin noncovalent complexes, complementing previously established methods. Host-guest complexes formed in solution are also stable for characterization by ESI in the gas phase. This paper reports the first investigations to characterize the stability of three inclusion complexes between beta-cyclodextrin (beta-CD) and three model "guest" molecules, by determining the cyclodextrin compound complex stability constant (K(st)) with the use of mass spectrometric studies. The relative signal intensity of the complexes were monitored in the positive ion mode by mixing each "guest" molecule with an up to 50-fold molar excess of betaCD. A novel linear equation, similar to Benesi-Hildebrand, was derived allowing the determination of K(st) for 11 stoichiometry in all complexes. These values were compared with the K(st) obtained by spectrophotometric experiments and they were evaluated to be slightly different, indicating the validity of the described method.