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This paper presents the results of reactivity controlled compression ignition (RCCI) combustion activated by hydrotreated vegetable oil (HVO). RCCI is a low-temperature combustion system that produces very low nitrogen oxide (NOx) and particulate matter (PM) emissions. HVO is a renewable fuel derived from waste materials. Its high cetane number, absence of aromatic hydrocarbons, low sulphur content, and lower carbon-to-hydrogen ratio position HVO as an attractive alternative to diesel fuel. Despite a higher reactivity gradient in RCCI combustion, HVO can positively influence exhaust emissions. The tests were conducted using a single-cylinder AVL 5402 research engine at a single operating point of 0.8 MPa indicated mean effective pressure and 1500 rpm. Detailed exhaust speciation was performed using AVL Sesam FTIR multi-component gas analyser, while a TSI EEPS 3090 particle size spectrometer provided PM characterisation. The comparative tests included two high-reactivity fuels: diesel and HVO. In both cases, methane was used as the low-reactivity fuel. At RCCI engine calibration for diesel-methane combustion, indicated thermal efficiency reached 44.2%, NOx emissions were 1.2 g/kWh, and PM emissions were 0.95 mg/kWh. However, replacing diesel with highly reactive HVO caused premature ignition, increasing peak temperatures while elevating NOx emissions to 9.7 g/kWh and reducing thermal efficiency. Diesel-like efficiency was restored when HVO injection was advanced by 15 crank angles, along with significantly lower NOx emissions (0.73 g/kWh) and satisfactory PM emissions of 1.46 mg/kWh. Simultaneously, methane slip was reduced by half. Furthermore, for RCCI with HVO, the methane fraction up to 90% was attainable without compromising emissions, contributing to a lower carbon footprint for RCCI technology.