In the present activity, dual fuel operation was investigated in a single cylinder research engine. Methane was injected in the intake manifold while the diesel was delivered via the standard injector directly into the engine. The aim is to study the effect of increasing methane concentration at constant injected diesel amount on both pollutant emissions and combustion evolution in an optically accessible engine. Emissions are in line with those previously published by other authors, it is noted no PM and constant NOx emissions. Moreover, a decrease of the brake specific CO emissions and an increase of the brake specific THC for the operating condition with the highest premixed ratio was detected. THC was mainly constituted by methane unburned hydrocarbons. Combustion resulted more or less stable. Moreover, via both UV-VIS spectroscopy and digital imaging, the spatial distribution of several species involved in the combustion process was analyzed. In particular, OH radical was recognized via chemiluminescence analysis and its integral intensity from UV digital imaging was compared with the ROHR of the several operating conditions investigated. Ignition delay is not affected by the premixed charge of methane. Moreover, the premixed combustion shows stronger OH radical emission, more than an order of magnitude, increasing the methane quantity in the burning charge.