The present work evaluates the effects of using blends of diesel fuel with two (straight) vegetable oils, viz sunflower and cottonseed, and their corresponding methyl ester biodiesels in proportions of 10% and 20% (by volume), on the performance and emissions of a fully instrumented, six-cylinder, turbocharged and after-cooled, heavy-duty direct injection (HDDI), Mercedes-Benz, bus diesel engine. The series of tests are conducted using each of the biofuels blends, with the engine working at (the same) three loads and two speeds, with measurements taken of fuel consumption and exhaust regulated emissions, i.e., smoke, nitrogen oxides (NOₓ), carbon monoxide (CO), and total unburned hydrocarbons (HC). Results of heat release combustion analysis of experimentally obtained cylinder pressure diagrams, together with the differing physical and chemical properties of these biofuels against those for the diesel fuel, which constitutes the baseline fuel, aid the correct interpretation of the observed engine behavior. With increasing percentage of all biofuels in the blends, reduction of smoke opacity (higher for the cottonseed oil or its biodiesel against their sunflower counterparts) is observed, slight increase of NOₓ, increase of HC emissions, increase of CO with the vegetable oil blends, and decrease with the biodiesel ones, while brake thermal efficiency remains practically the same.