Environmental issues such as emissions of pollutants and possible future petroleum shortages has led to intensified research efforts into sustainable alternatives for the use of diesel fuel in ignition engines. Straight vegetable oils provide cleaner burning and renewable alternatives, however their comparatively high viscosities may contribute to increased engine wear. Lowering the viscosity can be achieved by both increasing the temperature of the oil and mixing it with diesel fuel. The aim of this study is to investigate the rheology of diesel fuel and vegetable oil mixtures at differing compositions with respect to temperature to determine a viscosity-temperature-composition relationship, which can be used for the design and optimisation of heating and fuel injection systems. A rotational rheometer with concentric cylinder geometry was employed to determine the rheological properties of blends based on soybean, olive, canola and peanut oils with a commercial diesel fuel at temperature ranging from 20 to 80°C. All samples tested exhibited time-independent Newtonian behaviour over concentration and the temperature ranges studied. A modified Arrhenius relationship was developed to predict the viscosity (dynamic and kinematic) of the mixtures as functions of temperature and composition. The activation energy terms were found to be dependent on the oil concentration but relatively insensitive to the type of vegetable oil used.