The 41 litre (2500 cu. in.) per cavity natural gas fueled industrial rotary engine manufactured by Ingersoll-Rand Co. has completed many hours of successful field operation. Occasional engine detonation has been observed when the fuel quality deteriorated due to the presence of liquid hydrocarbons in the natural gas fuel. Such a condition could cause structural damage particularly to the apex seals. Detonation characteristics of this engine are discussed and the experimental and analytical results presented. The detonation limit of the engine for a given fuel can be expressed as a function of the firing pressure and the adiabatic compression temperature of the end gas. A method for estimating the stagnation detonation pressure is presented based on engine data obtained on an Ingersoll-Rand IR-2500 engine operating under incipient detonation conditions. For natural gas fueled engines the stagnation detonation pressure can exceed 32.4 MPa (4700 psia). Operating field experience has proven that for naturally aspirated rotary engines the apex seal designs based on the estimated pressure loading resulted in successful engine design and longevity.