This paper begins with a brief introduction to the operating principles of a split-cycle engine that utilizes a valved, intermediary volume to connect the two engine cylinders. Results from experimental testing of the engine fueled with pure methane are presented with a particular emphasis on the combustion duration and phasing. Two different methods of analysing the combustion duration – the mass fraction burn (MFB) and normalized pressure ratio (PRN) – are given. Testing was performed at wide open throttle (WOT) for engine speeds ranging from 850–1200 rpm, and air-fuel equivalence ratios from 0.8–1.0. The results indicate that the main combustion duration is very rapid for all conditions tested, despite late combustion phasing. Changes in spark timing were shown to have a considerable impact on IMEP but did not greatly effect the burn duration. Cyclic variability of IMEP was found to be less than 4% for all cases, except when operation was leaner than ø = 0.85, indicating good combustion stability.