It is important to use hydrous ethanol in the engine due to the removal of water from fermented products of ethanol consumes lots of energy. In this study, the effect of hydrous ethanol on the combustion and emissions was investigated in dual-fuel reactivity controlled compression ignition (RCCI) mode with port-injected hydrous ethanol and direct-injected diesel. The purity of ethanol was changed from 60% to 100% in 10% increment by adding different volume water in pure ethanol. Meantime, the volume ratio of port-injected pure ethanol among the total fuel (abbreviated as RE) was set to 60% and 80%. Results show that the higher RE causes the decrease of the combustion efficiency and indicated thermal efficiency. With the decline of ethanol purity, the combustion efficiency decreases and the maximum pressure rise rate (MPRR) shows a tendency of firstly going down and then rising. The thermal efficiency has a small change in the ethanol purity range of 80%-100%, whereas the thermal efficiency is reduced remarkably at the ethanol purity of 60%. The reduction of ethanol purity can reduce NOx emissions but CO and THC emissions increase. Adjusting the intake temperature, EGR rate and injection pressure will improve the combustion and emissions. (C) 2017 Elsevier Ltd. All rights reserved.