The kinetics of the addition reaction of O-2 to the ethyl radical has been investigated as a function of temperature (259-425 K) and pressure (1.5-60 bar) using the muon spin relaxation technique in longitudinal magnetic fields. Within this temperature range at 1.5 bar, the chemical reaction is represented by an Arrhenius rate law with an activation energy of -4.4(4) kJ mol(-1) and an apparent frequency factor of 1.3(2) x 10(-12) cm(3) molecule(-1) s(-1). The high-pressure limit of the rate constant at 294 K amounts to k(ch)(infinity) = 8.7(8) x 10(-12) cm(3) molecule(-1) s(-1). Within error, this limit has been reached at 1.5 bar. The rate coefficient for spin exchange, k(ex) = 2.8(2) x 10(-10) cm(3) molecule(-1) s(-1), is collision controlled. The results for k(ch) agree well with experimental literature values, but the temperature dependence is more pronounced than that predicted on the basis of a RRKM extrapolation from low-pressure data. The theoretical basis for the analysis of experimental data is given, and the results are discussed.