Recent studies have shown that the reaction between acetyl radicals and 0, at low pressures leads to the direct fast formation of OH radical, but the nature of co-products is controversial. Laser photolysis coupled to TDLAS (10-200 Torr, 298 K) has been employed to directly monitor two possible co-products of this reaction, CO and formaldehyde. Only CH2O has been detected ill yield of 0.29 +/- 0.13, but with time constants much slow than the OH formation under these conditions; the observed CH2O-time profiles are compatible with the known mechanism of peroxyacetyl secondary reactions. (c) 2005 Elsevier B.V. All rights reserved.