Abstract: The main object of this work is to demonstrate whether the ion exchange method can be used to determine stability constants of coordination compounds in non-aqueous solution. For this purpose, the determination of the stability constants of coordination compounds of 18-crown-6 with lanthanum (Ⅲ) by ion exchange method was investigated. The result (log β = 3.36) obtained is in good agreement with values by pNa and calorimetry method. This demonstrates that the ion exchange method is reliable and can be used to study stability of coordination compounds in non-aqueous solvents such as methanol.