Abs. rate consts. and the CH2:CHO.bul. (I) product yield for the title reaction were detd. using a combined laser photolysis-resonance fluorescence (laser-induced fluorescence) technique. The rate const. for HO.bul. decay was k1 = (1.1 +- 0.4) * 10-11 exp[-(1460 +- 150)K/T] cm3/s at 298-435 K. The branching ratio for I formation at 298 K was 0.08 and 0.23 at 13 and 80 mbar, resp. A potential-energy scheme accounting for reagent ring opening and I formation is presented.