The reaction C2H5 + O-2 has been studied by steady-state photolysis of mixtures containing Cl-2, C2H6 and O-2 over the temperature range 298-680 K at a constant density of 6.8 x 10(18) molecules cm(-1). Limited experiments were also performed as a function of pressure (200-1300 Torr) at four temperatures. After UV irradiation, the mixtures were analyzed by GC/MS to determine the product yields. The yield of C2H4 increases slowly between 298 and 450 K (E-a similar to 1 kcal mol(-1)) and then increases sharply (E-a similar to 25 kcal mol(-1)) reaching a yield of 100(+/-10)% of the O-2 reaction channel by 630 K. For T < 450 K, the C2H4 yield depends on the inverse of the pressure, indicating that the ethylene is formed via a chemically activated (C2H5O2*) radical as has been observed previously. Above 500 K, the C2H4 yield is independent of pressure indicating that a new channel has opened. This is confirmed by the observation that the ratio beta (= C2H4/C2H5Cl) increases sharply (from 0.8 to 3.5) between 450 and 500 K. If the C2H5 radical remained the sole source of C2H4 (via C2H5O2*) throughout the entire temperature range, no sharp break in beta would occur. The very small yield of ethylene oxide (2.5% at 660 K) and the excellent carbon balance between C2H6 consumed and products formed above 530 K support the formation of C2H4 at elevated temperature via both the thermally activated, concerted path (C2H5O2 --> C2H4 + HO2) proposed recently and the chemically activated (C2H5O2*) path. The former reaction occurs from a stabilized ethylperoxy radical without passing through a stable hydroperoxyethyl radical. Phenomenological rate constants at the experimental density are presented for the chemically and thermally activated paths to C2H4 formation. The data also indicate that the formation of C2H4 by direct H-atom abstraction is negligible under the conditions of these experiments [k(abs)(700 K) < 1 x 10(-13) cm(3) molecule(-1) s(-1)].