The reaction of H atoms with electronically excited O/sub 2/, H(/sup 2/S/sub 1/2/) + O/sub 2/(/sup 1/..delta../sub g/) ..-->.. OH(/sup 2/PI) + O(/sup 3/P) (1a), was studied in an isothermal flow reactor in the temperature and pressure ranges of 295 less than or equal to T/K less than or equal to 423 and 70 less than or equal to p/Pa less than or equal to 1070, respectively, with He as the main carrier gas. The concentration profiles of H and O atoms were followed by resonance absorption and those of OH radicals by LIF. The O/sub 2/(/sup 1/..delta../sub g/) molecules which were present in an excess over H atoms were generated in a microwave discharge in O/sub 2/(/sup 3/..sigma../sub g//sup -/) and their absolute concentrations determined by photoionization. At a pressure of p = 130 Pa, an Arrhenius expression of k/sub 1a/ (T) = 1.1 x 10/sup 11/ exp(-(13 +/- 2 Kj mol/sup -1/)/RT) cm/sup 3//(mol s) was obtained. It is proved that energy transfer from O/sub 2/(/sup 1/..delta../sub g/) to HO/sub 2/(X,A) is significant for the understanding of the H + O/sub 2/(/sup 1/..delta../sub g/), (/sup 3/..sigma../sub g//sup -/) system. The result of k/sub 1a/(T) and the reaction mechanismmore » including the reactions of O/sub 2/(/sup 3/..sigma../sub g//sup -/) and the energy-transfer processes O/sub 2/(/sup 1/..delta../sub g/) + HO/sub 2/(X,A) are discussed. 29 references, 8 figures, 3 tables.« less