Heat capacities of aqueous NaOH/NaAl(OH)(4) solutions at total stoichiometric ionic strengths of up to 6 mol kg(-1) and with up to 60 mol % substitution of OH- by AI(OH)(4)(-) have been measured at 25 degreesC using a Picker flow calorimeter. The mean apparent molar heat capacities (CN) of the NaOH/NaAl(OH)(4) mixtures were linear with respect to Al(OH)(4)(-) substitution, consistent with Young's rule. This enabled C, values to be derived for the hypothetical pure NaAl(OH)(4)(aq) solutions. The results so obtained are well described by a Pitzer model adapted from the literature, even up to high concentrations.