Nitrous oxide has been a compound of interest for the past several decades due to its exothermic decomposition. Currently nitrous oxide is used in a variety of industrial applications and is being investigated as a potential rocket propellant for next stage thrusters. Also, concerns about the safety of nitrous oxide have arisen in the nuclear industry, as it has been observed tha t stored nuclear waste generates high pressures of nitrous oxide with various gaseous fuels. Therefore, understanding the combustion characteristics, particularly under high pressure, of mixtures of fuel with nitrous oxide is highly important. The present work investigates the high pressure combustion characteristics of a stoichiometric mixture of ethylene and nitrous oxide (C 2 H 4 + 6N 2 O) in an alloy steel vessel with a 4 in. inner diameter and an internal length of 24.5 in. The combustion is initiated by energizing either a nichrome wire or an electric match connected to leads in the vessel. The experimental setup accommodates four high pressure (100,000 psia) transducers to measure pressure peaks at different points along the length of the vessel during the propagation of the combustion wave. Based on the time instants at which these peak pressures are recorded by the transducers it is possible to estimate the propagation speed of the combustion wave. The focus of this series of experiments is to investigate the dependence of combustion pressures, propagation speeds, and deflagration-to-detonation transition (DDT) on initial pressures of the above mixture of ethylene and nitrous oxide. Experiments were carried out at initial pressures of 100 psia, 125 psia, 150 psia, 200 psia, 337 psia, and 500 psia,. The preliminary 100 psia initial pressure tests were used to assess the correct functioning of all instrumentation. Overdriven detonations of different strengths were observed during each of the above tests. An overdriven detonation refers to the detonation process in which the main detonation parameters, such as detonation pressure and propagation velocity, exceed the corresponding CJ values. Along with this observation the transducers recorded elevated pressures, which were magnitudes in excess of the CJ values. The surprising experimental results are discussed and explained using theoretical considerations of flame acceleration, detonation, and DDT mechanisms.