This article describes the effects of hydroxylated biodiesel (castor oil methyl ester – COME) on the properties, combustion, and emissions of butanol–diesel blends used within compression ignition engines. The study was conducted to investigate the influence of COME as a means of increasing the butanol concentration in a stable butanol–diesel blend. Tests were compared with baseline experiments using rapeseed methyl esters (RME). A clear benefit in terms of the trade-off between NOX and soot emissions with respect to ULSD and biodiesel–diesel blends with the same oxygen content was obtained from the combination of biodiesel and butanol, while there was no penalty in regulated gaseous carbonaceous emissions. From the comparison between the biodiesel fuels used in this work, COME improved some of the properties (for example lubricity, density and viscosity) of butanol–diesel blends with respect to RME. The existence of hydroxyl group in COME also reduced further soot emissions and decreased soot activation energy.