This method is for determining the concentration of ammonium ion in aqueous solutions by Ion Chromatography (IC). The limit of quantitation of the method is 0.0001 mass-% (1 mass-ppm). Sodium and potassium interfere with the analysis when they are present at concentrations greater than 0.1 mass-%. Acidic samples that contain greater than 50-mM hydronium ion concentration (pH less than 1.3) deteriorate the performance of the analysis (through loss of resolution). Organic contamination should also be kept to a minimum (to lengthen column life). Sample matrix effects can be avoided or minimized by sample dilution, but only at the expense of the analysis limit of quantitation. When used in conjunction with UOP Method 958 (an extraction procedure), ammonia can also be determined in hydrocarbon samples.