Measuring calcium in saliva is important because it is likely that calcium content in saliva is connected to a number of diseases. Calcium is present in saliva as free Ca2 + ions and as complexes with inorganic and biochemical components, and both forms are of medical interest. Two issues are addressed here for the first time. Firstly, how much strong electrolyte must be added to saliva sample to set the value of Ca2+ activity coefficient, so Ca2+ concentration can be calculated from Ca2+ ion activity measured by a calcium selective electrode (Ca-ISE). Secondly, to what pH value must saliva sample be acidified to ensure release of Ca2+ from, at least, complexes with inorganic anions: to estimate also the total calcium content in samples. Model calculations accounting for Ca2+ complexation with inorganic anions: phosphates and carbonates (25 0C) show that addition of 0.1 M NaCl is enough to set the ionic strength (IS) at 0.144 ± 0.018 M and Ca2+ activity coefficient (γCa2+) at 0.385 ± 0.015 (pH 7), while over pH 7–2 IS is 0.142 ± 0.020 M, γCa2+ is 0.386 ± 0.018, and acidification to pH 4 is enough to release calcium from inorganic complexes. Experimental results in model solutions are consistent with calculations, however, ISE underestimate total calcium content in real samples measured by AES. Preliminary data suggest that this difference is due to complexation of Ca2+ ions by biochemical components of saliva, primarily by mucin. Thus, concentration of Ca2+ can be measured by Ca-ISE, however quantification of total calcium content by Ca-ISE appears less reliable.