Dual-fuel combustion using liquid fuels with differing reactivity has been shown to achieve low-temperature combustion with moderate peak pressure rise rates, low soot and NOx emissions, and high indicated efficiency. Varying fractions of gasoline-type and diesel-type fuels enable operation across a range of low- and mid-load operating conditions. Expanding the operating range to cover the full operating range of a heavy-duty diesel engine, while maintaining the efficiency and emissions benefits, is a key objective. With dissimilar properties of the two utilized fuels lying at the heart of the dual-fuel concept, a tool for enabling this load range expansion is altering the properties of the two test fuels - this study focuses on altering the reactivity of the diesel fuel component.Tests were conducted on a 13L six-cylinder heavy-duty diesel engine modified to run dual-fuel combustion with port gasoline injection to supplement the direct diesel injection. A single diesel injection strategy was used across a range of operating points. Diesel fuels with varying cetane number were tested, including a ULSD certification fuel; a low cetane, low aromatics, high volatility fuel from the Fuels for Advanced Combustion Engines matrix (FACE #1); and a high cetane Fischer-Tropsch diesel fuel.Decreasing diesel fuel cetane number reduces overall mixture reactivity and extends the mixing time prior to onset of combustion, reducing soot emissions and shifting the overall operating window to a higher load range. Stable operation with high gasoline fractions across a wider load range is enabled by the high cetane fuel.