The aim of this work was to investigate: the intersolubility of mixtures of rapeseed oil methyl esters, diesel fuel and ethanol; to determine the dependence of solubility upon temperature and finally to evaluate emissions of exhaust gases of these stable fuel mixtures. Bearing in mind that the cloud point is an important parameter of diesel fuel, the variation of solubility of a tri-component rapeseed oil methyl ester-diesel fuel-ethanol (RME-D-E) system at temperature (20; 0; -10 degrees C) was also investigated. It was found that temperature decrease causes the R-ME-D-E system solubility limits to become narrow. Solubility investigations allowed to determine the optimal solubility limits and select mixtures containing 6.9-25.7% of oxygen for engine tests. The highest oxygen content in biodiesel fuel permitting the engine to work normally at 2000 and 1200 min(-1) was 19.5%. The lowest concentration of PAH and smoke index of exhaust gases were deter-mined when fuel mixtures contained 19.5% of oxygen. The CO concentration depended on the rotational speed and varied from 10.7% to 16.8%. Apparently, optimal diesel fuel on this basis will contain from 15% to 19% of oxygen. (c) 2006 Elsevier Ltd. All rights reserved.