The thermodynamic equilibrium constants for the reactions between the cationic surfactant, ethylhexadecyldimethylammonium bromide, EHDA-Br, and various anions were determined using spectrophotometric and specific ion electrode measurements. The sequence of stability of EHDA+in the presence of the anions X−studied is Br−> F−> H2PO4−> NO3−> C6H5O−> I−. The sequence of stability of EHDA-X is the reverse of this. The EHDA+stability sequence is the same as the order of selectivity expected during foam fractionation from published results for Br−, I−, NO3−, and for H2PO4−and C6H5O−, i.e., I−preferred over NO3−, which is preferred over Br−, etc. The stability and selectivity sequences are interrelated by the steric hindrance of EHDA+in the presence of the anions X−at the bubble surface.