The efficiency of a batch microwave-assisted ammonia heating system was investigated as pretreatment for sweet sorghum bagasse and its effect on porosity, chemical composition, particle size, enzymatic hydrolysis and fermentation into ethanol evaluated. Sorghum bagasse, fractionated into three particle size groups (9.5-18, 4-6 and 1-2 mm), was pretreated with ammonium hydroxide (28% v/v solution) and water at a ratio of 1:0.5:8 at 100, 115, 130, 145 and 160 degrees C for 1 h. Simon's stain method revealed an increase in the porosity of the biomass compared to untreated biomass. The most lignin removal (46%) was observed at 160 degrees C. About 90% of the cellulose and 73% of the hemicellulose remained within the bagasse. The best glucose yields and ethanol yields (from glucose only) among all different pretreatment conditions averaged 42/100 g dry biomass and 21/100 g dry biomass, respectively with 1-2 mm sorghum bagasse pretreated at 130 degrees C for 1 h. Published by Elsevier Ltd.