Montelukast, a leukotriene receptor antagonist (LTRA) is used to prevent an asthmatic attack, shortness of breath and wheezing. As intravenous therapy, Calcium chloride (fused) is used to treat hypocalcemia. Using spectrophotometry, an in vitro study of the interaction between Montelukast sodium and Calcium chloride (fused) was conducted at pH 7.4 and pH 2.4 in aqueous systems at 37 ± 0.5 °C. A reverse V-shaped curve was found from the Job’s plot indicating a strong kinetics between Montelukast sodium and Calcium chloride. The stability constant was obtained from Ardon’s plot for the complexation at both pH values (7.4 and 2.4), which indicates that Montelukast sodium and Calcium chloride relatively form a stable complex at pH 7.4. Therefore, concomitant administration of Montelukast sodium and Calcium chloride (fused) needed careful consideration since there is a possibility of forming a complex which in turn reduces the therapeutic activity.