Low-temperature heat capacity of heulandite Na0.365K0.059Ca0.861Al2.138Si6.860O18. 617H(2)O was measured in an adiabatic vacuum calorimeter. The results obtained were compared with those for heulandite and clinoptilolite from literature. The errors in measurements of the water content are shown to be the main reason of discrepancies among the data on heat capacities for the zeolites. The random error can be as high as 1.1%. The greatest systematic error of 2.5% was shown to be in the heat capacities of clinoptilolite. Unusual temperature dependence has been recognized in published data on heat capacity of clinoptilolite, as if free electrons are there in a silicate. Heat capacity of four different species of heulandite-clinoptilolite series are known. However, the data published do not enable one to recognize the relationship between heat capacity and a composition.