The factorial design is used to study interferences of [K+] and pH during ammonium determi nation by NH4-ISE. It is concluded that as result of a negative pH-[K+] interactive interference the positive interference of [K+] may be diminished. The optimal pH for NH4+ determination is 7.4. A complete procedure is presented to diminish the interferences of [K+] and pH. Parallelly an indirect method is used for the determination of total ammonium in urine.