Magnesium trisilicate mixture BP, a liquid antacid, contains three active components; magnesium trisilicate BP (MT) (5% (w/v)), light magnesium carbonate BP (LMC) (5% (w/v)) and sodium bicarbonate BP (SB) (5% (w/v)). The role of each component in the mixture is not well understood although each contributes to the overall acid neutralising capacity (ANC) of the product, the standard measure by which antacid performance is rated. Previous work has suggested that the inclusion of magnesium trisilicate BP in the mixture is unwarranted [Int. J. Pharm., 29 (1986) 253], because of its extremely slow reaction with hydrochloric acid. In light of these observations, there is a need to study the basic mechanisms of neutralisation of the components of the mixture. Since the heat change associated with simple mono-protic acid–base neutralisation is approximately −57 kJ mol−1 [Chemistry Data Book, John Murray, London, 1982], calorimetry may be used to study the reactions. In this work, the relative contributions to the ANC of the product of each component were measured using solution calorimetry. It was found that light magnesium carbonate BP contributed most to the overall ANC of the product, sodium bicarbonate BP the least. Magnesium trisilicate BP was found to neutralise acid over an extended time period, and contributed most to the duration of action of the dose. Moreover, it appears that light magnesium carbonate BP and magnesium trisilicate BP in combination exhibit a greater than expected ANC, showing positive synergy.