The aim of the present work is the modification of the standard indophenol blue method in order to be feasible the determination of ammonium in geothermal water samples with high mineral content. Several pretreatment techniques were studied and evaluated in order to neutralise the pH of all solutions (blank, sample, standards) and minimise the interferences from salts. The Ultrasonic bath followed by mechanical stirring and vacuum filtration seems to be the most efficient pretreatment technique with almost 100% recovery of . Also there was an optimisation in the main procedure of the method in order to gain in sensitivity. The proposed method was compared with the standard ion chromatography and spectrophotometric indophenol blue methods and a t-test was performed. The RSD% values (n = 4) of the proposed spectrophotometric method were below 3.5% and the LOD of was 0.026 mg L⁻¹. The proposed method was applied to several geothermal water samples from regions of Greece.