In this paper, selected physicochemical properties (such as lower heating value (LHV), kinematic viscosity, density, lubricity, cold filter plugging point (CFPP), and cetane number (CN), as well as miscibility of diethyl ether (DEE)/diesel oil blends) were experimentally determined. DEE was added to diesel oil, meeting the requirements of European standard PN-EN 590, in volumetric proportions of 5, 10, 15, and 20 vol %. The LHV, kinematic viscosity, and density of the blends were lower than the values obtained for the tested diesel oil. Especially, it was found that DEE has significant influence on diesel oil viscosity. The addition of merely 5% DEE to diesel oil decreased its viscosity by 26%. It was shown that the lubricity of all tested blends is reduced, but not as significantly as viscosity. It was found that the temperature of the CFPP decreased when DEE was added to diesel oil. The results show that miscibility of all tested fuel blends is excellent in a wide range of temperature changes. The engine experiments were carried out on two different diesel engines. CN values of the tested blends were determined using a Waukesha Co. research engine, according to the procedure presented in European standard EN ISO 5165:2003. It was observed that the addition of DEE slightly increased the CN of the diesel oil. However, it did not have any influence on shortening of the ignition delay period in the tested AD3.152 diesel engine with direct fuel injection.