Binding constants for tetraphenylborate and tetraphenylphosphonium ions (Ph4B- and Ph4P+) to cyclodextrins (CDs) to give 1.1 host-guest complexes have been measured using capillary electrophoresis. Mobilities of the ions as a function of gamma -CD concentration give binding constants, K, of 1.08 x10(5) M-1 for Ph4B- and 0.6 x 10(1) M-1 for Ph4P+. This dramatic difference of four orders of magnitude in binding constants is not seen with beta -CD (K = 7.7 x 10(1) M-1 for Ph4B- and 3.7 x 10(1) M-1 for Ph4P+) or dimethyl (DM)-beta -CD (K = 46 x 10(1) M-1 for Ph4B-1 and 7.7 x 10(1) M-1 for Ph4P+). The crystal and hydrodynamic radii of the ions, the latter calculated from their absolute mobilities, indicate that Ph4B- is smaller than the gamma -CD cavity, whereas Ph4P+ is approximately the gamma -CD cavity size. Results suggest that Ph4B- fits exactly into a gamma -CD cavity, with hydrophobic contacts involving several of the phenyl rings, whereas Ph4P+ is too large to enable these multiple contacts to be made. When only a single phenyl ring can fit into the CD cavity, binding strengths are in the order DM-beta -CD > beta -CD > gamma -CD and Ph4B- > Ph4P+. Measurement of electrophoretic mobilities of the complexes shows that their hydrodynamic radii are in the order gamma -CD < -CD similar to DM-beta -CD for PR4B- and gamma -CD > beta -CD similar to DM-beta -CD for Ph4P+.