Higher alcohols such as butanol isomers have received remarkable attention as alternative fuels for compression ignition engines because of their great potential as blending components in mixtures with biodiesel and diesel resulting in significant reduction in greenhouse gases and harmful pollutants. In this context, the influence of temperature and pressure on the density of used cooking oil biodiesel, 1-butanol, isobutanol, and biodiesel mixtures with different levels of isobutanol (0.0500, 0.0941, and 0.1397 by mass) was assessed over the temperature range of 298–343 K and pressures up to 30 MPa. For this purpose, a vibrating tube densimeter was calibrated using water and n -octane. The average expanded uncertainty of measured density at a confidence level of 0.95 ( k = 2) was estimated to be 0.80 kg m –3 . The densities of all the systems were represented by the Tait equation of state with AARD % (0.01–0.035%) including the results for the new Tait equation for the biodiesel + isobutanol blends. When isobutanol is added to biodiesel to form a 14% by mass of alcohol, the density of mixture decreases 11 kg.m –3 . Thermal expansivity and isothermal compressibility increase with maximum deviations of 9.6 % and 7.8 % at 343 K and atmospheric pressure.