Abstract The isomerizations of bicyclo[5.4.0]undecene-(1,7) (IV), spiro[5.5]undecene-1 (V), and 1-cyclopentylcyclohexene (VI) were examined with equivalent amounts of perchloric acid in acetic acid or in ethanol at 75°C. In acetic acid, IV, V, and VI all rearranged to form spiro[5.5]-undecan-1-yl acetate (VII) and spiro[5.5]undecan-2-yl acetate (VIII), together with the three olefins IV, V, and VI. No rearranged product has been obtained by the same procedure in ethanol.