Oxa-dibenzocyclooctynes (ODIBO, 2a-c) are prepared by photochemical decarbonylation of corresponding cyclopropenones (photo-ODIBO, 1a-c). While photo-ODIBO does not react with azides, ODIBO is one of the most reactive cyclooctynes exhibiting rates of cycloaddition over 45 M(-1) s(-1) in aqueous solutions. ODIBO is stable under ambient conditions and has low reactivity towards thiols. Photo-ODIBO survives heating up to 160 °C and does not react with thiols.