Interactions between the β-cyclodextrin and the monosulfonated isomers of triphenylphosphine oxide were investigated in aqueous solution by NMR, UV-vis and ESI mass spectroscopy. Titration and continuous variation plots obtained from ³¹P and ¹H NMR data indicate the formation of 1 ∶ 1 inclusion complexes. The structures of these inclusion complexes were proposed from T-ROESY experiments. All inclusion complexes were enthalpy stabilized, but entropy destabilized. Formation of such complexes cannot decrease the efficiency of cyclodextrins in aqueous-phase organometallic catalysis.