Chiral spirobinaphtopyrans 1-9 were synthesised using the acid catalyzed aldol condensation of 2-hydroxy-1-naphthaldehyde or 1-hydroxy-2-naphthaldehyde with the appropriate ketone and subsequent reaction of the isolated pyrylium salt with base. Separation or enrichment of enantiomers was accomplished by low-pressure liquid chromatography (LPLC) on triacetylcellulose, tribenzoylcellulose, or (+)-poly(triphenylmethyl methacrylate) on SiO 2 . The barriers to thermal racemization were determined by dynamic 1 H NMR measurements and by on-line or off-line measurements of the enriched enantiomers after LPLC. Barriers to thermal racemization for reversible cleavage of the C spiro -O bond in 1-9 were found to be in the range 85-105 kJ/mol and are rationalized by steric and electronic effects. The ⌬G values decrease with decreasing length of the bridge or decreasing size of substituents in the 3 and 3Ј positions. Therefore, ⌬G for 7>9>6, 3>2, and 4>1. Geometrical data from X-ray crystal structure analysis show that the mean value of the C spiro -O bond length in the spirobibenzopyran 14 is shorter, i.e., the bond is stronger than that in the spirobinaphtho derivatives 4, 3, and 6. This bond-length shortening is explained by the weaker conjugation of the oxygen -lone electron pair with the -system of the benzene ring in 14 compared to the naphthalene moiety in 3, 4, and 6.