Pure ethylene oxide (EO) vapor may propagate decomposition flames through pipe above certain minimum conditions of temperature, pressure and pipe diameter. Flame propagation was studied in both closed and vented 5 cm (2-inch) pipe and closed 30 cm (12-inch) pipe. Flame progression in closed pipe was irregular and proceeded in pulsed stages. A possible mechanism involves preferential flame propagation at the pipe roof accompanied by periodic autodecomposition of EO accumulated in hot products behind the flame front, such accumulation probably being augmented by liquid EO condensed on the pipe walls ahead of the expanding flame system. Flames propagated 15 m (50 ft) through horizontal 5 cm pipe at 70°C and initial pressures ≥ 4.3 bar (62 psia). In a series of 30 cm pipe tests employing low-energy ignition and otherwise increasingly severe conditions, a deflagration-to-detonation transition (DDT) occurred, partially destroying the test equipment. A new test facility was set up to confirm the ability of EO to propagate detonations in 30 cm pipe and to further investigate the phenomenon. Two EO detonations at 2.9 bar and one at 3.5 bar were directly initiated via the strong shocks from hydrogen-oxygen detonations. Based on a spectrum analysis of the pressure histories, the two detonations at 2.9 bar were probably marginal and propagated in a single spin detonation mode. At 3.5 bar, the pressure history suggests that the detonation propagated in a two-head detonation mode near the end of the 24 m test section.