This paper investigates the formation of the pollutant nitric oxides (NOx) in the low-nitrogen (N-2) environment of methane oxycombustion in a spark-ignited (SI) internal combustion engine. Working fluid composition, N-2 concentration, O-2 concentration, compression ratio (CR) and spark-timing have been investigated to evaluate the feasibility of operating such a system below NOx regulation levels without after-treatment systems. NOx emissions in g/kW h are shown under equivalent CR, intake temperature, and indicated mean effective pressure (IMEP) at maximum brake torque spark-timing, to have an approximately linear dependence on N-2 concentration from no N-2 to normal air combustion. At a given N-2 concentration, NOx emissions were found to be adversely correlated with power, thermal efficiency, and the coefficient of variation of 1MEP. It was found that with 2-3% N-2 by volume in the working fluid, it was possible to reduce NOx emissions to satisfy regulation levels, but this corresponds to non-ideal engine performance in other metrics. Satisfying regulations while operating at the maximum thermal efficiency required the N-2 concentration be reduced to 1-2% by volume. The system was simulated using an AVL Boost model, with results indicating that the increasing NOx concentrations at higher O-2 cases and earlier spark-timings can largely be attributed to higher burned-gas temperatures. An additional simulation utilizing CHEMKIN and the GRI 3.0 mechanism was used to estimate NOx formation, and with results indicating that air-calibrated NO mechanisms maintain reasonable accuracy in low-N-2 environments. (C) 2013 Elsevier Ltd. All rights reserved.