The thermal isomerizations of 1′,3′,3′-trimethyl-spiro[2H-1-benzopyran-2,2′-indoline]-6-sulphonic acid (6-SOmath image-SP) between its spiro (SP) and merocyanine (MC) forms were followed spectrophotometrically in the presence of cyclodextrins (β- and γ-CDs) at various pressures. From the pressure dependence of the isomerization rate, the activation volumes for the thermal isomerization of 6-SOmath image-SP were estimated to be 17·1 cm3 mol−1 for MC [RIGHTWARDS ARROW] SP and 8·5 cm3 mol−1 for SP [RIGHTWARDS ARROW] MC. The equilibrium constants (K) for the inclusion complex formation of CDs with 6-SOmath image-SP were determined at various pressures. It was found that the stability of the 1:1 inclusion complex between 6-SOmath image-SP and CDs decreases in the order β– > γ– > α-CD. From the pressure dependence of K, the reaction volumes were estimated to be −0·9 cm3 mol−1 for the 6-SOmath image-SP–β-CD system and 4·0 cm3 mol−1 for the 6-SOmath image-SP–γ-CD system. Based on the activation volumes, the structure of the transition state for the thermal isomerization is discussed and an explanation for the effect of CDs on the reaction rate is given. The difference in the reaction volumes for the inclusion complex formation of β- and γ-CDs is explained in terms of the difference in the number of water molecules excluded from the CD cavity.