The objective of this paper is to study the suitability of several laboratory methods for calculating the thermo-chemical corrections (TCCs) of acid formation, which generate extra heating during the combustion of biomass in a calorimetric bomb. Three methods for calculating TCCs are considered in this paper: two of them are based on the titration of acidity, and the third one is based on analysing anion (nitrate, sulphate and chloride) content. This work compares the three TCCs with the composition of one coal and eight biomass samples. TCCs based on titration are not recommended because of the neutralisation of the nitric and sulphuric acids produced during the combustion of the biomass. The compounds contained in the biomass ash, such as CaCO3, are probably the cause of this neutralisation. The measurement of nitrate content in order to calculate the TCC of the formation of nitric acid was found to be the most suitable and accurate. The formation of aqueous hydrochloric acid is improbable in the biofuels considered. (c) 2007 Elsevier B.V. All rights reserved.