Gypsum is a sparingly soluble salt, often precipitates in some industrial equipment and imposes instrumental damage. Prediction of phase behavior and scaling tendency of gypsum requires recognition of all ionic interactions in an electrolyte aqueous solution. In this respect, Ion Selective Electrode (ISE) is progressively applied in industry and laboratory as a fast and online monitoring tool to measure concentration of individual ions. This study is an attempt to advance the current application of ISEs for reliable measurement of concentration of ions with more precision. The solubility of gypsum in the aqueous solution of K2SO4 and NaCl is measured at the different temperatures. We carried out a two-part experimental approach that at the first step we focus on effect of the ionic interactions on activity of Ca2+ in multi-electrolyte solutions. Following determination of the ionic interaction parameters, the calcium ISE is used to measure the concentration of Ca2+ in a saturated solution of gypsum so that the influence of the other ions on the gypsum formation is investigated. The Khoshkbarchi and Vera's (KV) and Lin and Lee (LL) models are applied to calculate the activity coefficient of Ca2+ and discussed in this work. (C) 2015 Elsevier B.V. All rights reserved.