A long-path FTIR spectroscopic study of the Cl-atom-initiated reactions of ethylene oxide (H2C-O-CH2) was carried out at 297 +/- 2 K in the photolysis (300 nm) of mixtures containing H2C-O-CH2 and Cl-2 in both the torr and millitorr ranges in 700 Torr of N-2 or O-2/N-2 diluent. In 700 Torr of N-2, the only primary product detected was H2C-O-CHCl, formed via (1) Cl + H2C-O-CH2 --> H2C-O-CH + HCl followed by (2) H2C-O-CH + Cl-2 --> H2C-O-CHCl + Cl. Thus, the cyclic oxiranyl radical H2C-O-CH formed in reaction 1 was sufficiently long-lived to react with Cl-2. An upper limit value of k(3) less than or equal to 2 x 10(4) s(-1) has been estimated for the rate constant of the possible oxiranyl-to-vinoxy isomerization : (3) H2C-O-CH --> CH2CHO. The H2C-O-CHCl yield decreased with increase in added O-2 due to the occurrence of reaction 4 : (4) H2C-O-CH + O-2 (+ M) --> H2C-O-C(OO)H (+ M). A value of k(2)/k(4) = 2.0 +/- 0.4 was derived from the O-2 dependence of the H2C-O-CHCl yield. In 700 Torr of air, the observed products included C-O-C bonded compounds HC(O)OCHO and CH2(OH)OCHO, and one-carbon species CO, CO2, HCHO, and HC(O)OH, but not C-C bonded products. The preferential formation of C-O-C rather than C-C bonded products suggests the predominant cleavage of the C-C bond rather than the C-O bond in a three membered ring precursor radical. The most likely candidate is the cyclic radical H2C-O-C(O)H formed via self-reaction, i.e., 2H(2)C-O-C(OO)H --> 2H(2)C-O-C(O)H + O-2. A detailed mechanism is proposed for the oxidation of the H2C-O-C(O)H radical leading to the formation of the observed products.