The paper presents research results of the AVL diesel engine powered with microemulsion fuels. In particular the emission of soot particles from the engine operated at stationary conditions was investigated. Necessary tests were carried out for two microemulsions containing 3.5 and 7 % by volume of distilled water dispersed in diesel fuel with necessary surfactants/cosurfactants, i.e. Crillet-6/Span-20. Both tested microemulsions contained similar water micelles size distribution with average dimension of 18 nm. Results of investigation confirmed that combustion of tested water – fuel microemulsion in the AVL research engine reduces emission of soot particles even by 50% in comparison with diesel fuel.