Composite additives of ethanol, acetonitrile, and methyl acetate were added to bio-oils to assess the effects of these additives on stability of bio-oil using D-optimal experimental mixture design (DMD). A systematic analysis of the influence of different proportions of composite additives on bio-oil properties such as viscosity, water content and pH was carried out. The results showed that bio-oil had a lower viscosity, water content and higher pH when the additive formulation consisted of 6.58 wt% ethanol, 1 wt% acetonitrile and 2.42 wt% methyl acetate that was determined to be an optimal composite additive (OCA). Experimental trials were performed to validate the models. A fast aging test at 80 degrees C for 24 h confirmed that the composite additive was better than single additives to improve stability of bio-oils. The viscosity of bio-oil with OCA was 27.25 mm(2)/s and was reduced by 33-47% compared to the bio-oil with a single additive; while reduced by 95% with no additives. GC-MS analysis showed that several compounds such as 4-hydroxy-Butanoic acid, p-Cresol, and Vanillin were virtually eliminated in bio-oil with addition of the composite additive. This research illustrated the beneficial effect of the composite additive for storage of bio-oils. (C) 2015 Elsevier Ltd. All rights reserved.