Inorganic acids, such as sulphuric acid, hydrochloric acid, and nitric acid are widely used for the pretreatment of lignocellulosic biomass for bioenergy production. In this study, the effect of different acids on the recovery of reducing sugar from coconut residues (coir and pith) mixed in different ratios was studied. The pretreatment conditions for different acids were optimized using response surface methodology (RSM). The independent variables, such as biomass ratio, time and acid concentration were considered for the optimization studies with reducing sugar as the dependent variable. The maximum recovery of reducing sugar (45%) from mixed biomass was observed during nitric acid (NA) pretreatment. The recovery of reducing sugar was lower for hydrochloric acid (HA) and sulphuric acid (SA). The lower yield was attributed to the possible formation of sugar degradation compounds during acid pretreatment. Therefore, NA pretreatment was found suitable for mixed biomass compared to other acids. Further studies are required to understand the effect of NA pretreatment through a detailed study of liquid hydrolysate and the introduction of the saccharification process. Mixed biomass benefits the biorefinery industries for sustainable bioenergy production. © The Authors.