The products obtained from the ArF laser photolysis of H2CCO in the presence of a large excess of NO at T = 298 K and p = 3.5 mbar in He bath gas were investigated in a tubular flow reactor by time-resolved mass spectrometry. The main reaction products observed in the presence of NO were HCNO and HCN. The measured yields, F-HCNO = 0.78 +/- 0.18 and Gamma(HCN) = 0.19 +/- 0.04, respectively, were attributed to the contributions of the reactions CH2 + NO --> HCNO + H (1a) CH2 + NO --> HCN + OH (1b) and HCCO + NO --> HCNO + CO (2a) (HCCO + NO --> HCN + CO2 (2b) The time-resolved measurements are compared to results of preceding end product analyses by Fourier transform IR spectroscopy. (C) 2002 Published by Elsevier Science B.V.