This manuscript presents a deep analysis of natural gas-hydrous ethanol dual-fuel combustion in a spark ignition engine with a modified compression ratio and an advanced intake valve opening strategy to promote intemal exhaust gas recirculation (iEGR). Both fuels were port-fuel injected and two different liquid fuel replacements by compressed natural gas (CNG) were tested (18 and 45% by energy). Combustion, performance and pollutant emissions were investigated under stoichiometric air-fuel conditions at 4 bar of net indicated mean effective pressure (IMEP) and 1800 rpm. Results show that dual-fuel mode improves fuel conversion efficiency when compared to CNG-only operation, due to lower carbon monoxide (CO) and unburnt hydrocarbons (HCs) emissions as well as to a better combustion phasing. The use of iEGR, despite deteriorating the combustion process as expected (ignition delay, combustion duration and combustion stability), improves efficiency because of a decrease in pumping losses and wall heat transfer, as well as pollutant emissions reduction. Nitrogen oxide (NOx) emissions increase with dual-fuel operation. However, the influence of iEGR was found to be more significant under dual-fuel mode than under single-fuel operation, with NOx reductions of up to 70% and fuel conversion efficiency improvements of around 2.5%.