Fractionation of lignocellulosic biomass is a primary step for converting multi-structure biomass to biofuels and other industrial products in integrated biorefinery processes. Here, the effects of homogeneous and heterogeneous acid promoters (HNO3-, HCl-, H2SO4-, and H3PO4-activated carbons) on Clean Fractionation (CF), a single-step aqueous-organosolv fractionation, of eucalyptus wood chips was studied and optimised. The optimised process contained 16.7% w/v biomass in a ternary mixture of methyl isobutyl ketone:methanol:water (25:42:33) with 5% AC-H3PO4 and incubated at 180 degrees C for 60 min. Under these conditions, 41.2 wt.% cellulose was obtained in enriched solid pulp with the average glucan content of 75.9%. The majority of the hemicelluloses was hydrolysed into the aqueous-methanol phase, which contained 17.8 wt.% monomeric xylose and xylooligomers while 13.7 wt.% lignin was separated into the organic phase. The heterogeneous acid-promoter process is a potent alternative to corrosive homogeneous acids for lignocellulose fractionation in integrated biorefineries. (C) 2013 Elsevier Ltd. All rights reserved.