As global energy demands continue to be met with ever evolving and stricter emissions requirements, natural gas (NG) has become a highly researched alternative to conventional fossil fuels in many industrial sectors. Transportation is one such field that can utilize the benefits of NG as a primary fuel for use in internal combustion engines (ICEs). In the context of heavy-duty on-highway transportation applications, diesel-ignited dual-fuel (DIDF) combustion of NG has been identified as a commercially viable alternative technology. Previous investigations of DIDF have examined the various trends present across the spectrum of DIDF operating space. However, in-cylinder processes are still not well understood and this investigation aims to further understanding in this area.An in-cylinder, local infrared absorption fuel concentration sensor is used to examine in-cylinder processes by comparison with previous optical and thermodynamic studies. A 2-litre single-cylinder research engine was operated at selected DIDF operating modes with significantly different fuel conversion mechanisms. Fuelling was achieved through port injection of CH4 and direct injection of diesel. The operating modes were specified by varying the relative pilot diesel injection mass and pressure along with CH4 injection mass. Additionally, an injection timing sweep of port injected CH4 was performed at a constant fuelling point. Through analysis of the measured fuel concentration and apparent heat release rate (AHRR), the combustion mode could be identified with greater certainty than using just pressure-based measurements alone.