The significance of the paper is confirmed by the need to replace petroleum motor fuels with fuels produced from alternative energy sources. Biofuels derived from various vegetable resources are considered as promising alternative fuels for diesel engines. These fuels offer significant advantages with respect to the renewability of their raw materials and good emission performances when burned in ICEs. The main problem of using vegetable oils as biofuels for diesel engines is their high viscosity. This problem can be resolved by using mixed biofuels with the addition of gasoline. The analysis of physico-chemical properties of petroleum diesel fuel (DF) and mixed biofuels containing petroleum DF, rapeseed oil (RO) and AI-80 automotive gasoline was conducted. Experimental studies of the D-245.12S diesel fuelled with these mixed fuels were carried out. The mixed fuels were prepared from 80% DF + 20% RO, from 75% DF + 20% RO + 5% AI-80, and from 70% DF + 20% RO + 10% AI-80. It was shown that the addition of gasoline to mixed biofuels could improve two main toxicity indicators of exhaust gases exhaust gases smoke and emissions of nitrogen oxides. The best emission performance was achieved for the mixture of 70% DF, 20% RO and 10% AI-80. When the diesel engine was switched from the mixture of 80% DF and 20% RO to the mixture of 70% DF, 20% RO and 10% AI-80, the exhaust gases smoke at maximum torque mode decreased from 17.5 to 14.5% on the Hartridge scale, i.e. by 17.1%. The specific emissions of nitrogen oxides decreased from 6.559 to 6.154 g/(kW·h), i.e. by 6.2%.