The products and rate constants for total physical and chemical quenching (k(q) + k(t)) of singlet oxygen (O-1(2)) by cis- and trans-cyclooctene were determined in two solvents of different polarity. Small amounts of cis-cyclooctene are produced during the reaction of the trans-isomer. in CDCl3 and acetone-d(6), k(q) + k(r) for cis-cyclooctene was 1.3 x 10(4) M(-1) s(-1) while the rate constants (k(r)) for the reaction of trans-cyclooctene were 2.3 x 10(4) and 3.4 x 10(4) M(-1) s(-1), respectively. Competition experiments with 2-methyl-2-pentene (2M2P) suggest a substantial contribution of physical quenching for the trans-alkene while the cis-alkene removes O-1(2) mostly by chemical reaction. The physical quenching of O-1(2) by trans-cyclooctene is explained by a perepoxide intermediate which can open to a zwitterion that can abstract an allylic hydrogen to give the 3-hydroperoxycyclooctene ene product or isomerize and lose O-2 to form cis-cyclooctene. A perepoxide intermediate can be trapped using triphenyl phosphite with trans-but not cis-cyclooctene. trans-Cyclooctene quenches C-3(70) with a rate constant of 3.4 x 10(5) M(-1) s(-1).