Ammonium attenuation in subsoils and groundwater is predominantly due to cation exchange and/or nitrification (biological oxidation) processes. These processes have been little studied in UK formations and this relative lack of information can result in reduced consistency and robustness in the assessment of risks posed by ammonium contamination arising from landfills, effluent soakaways, contaminated sites and other sources. A review of ammonium fate and transport in the subsurface has been completed and guidance developed on the key processes that contribute to attenuation. The amount of relevant literature is small but sufficient to provide indicative ranges of partition coefficients and biological nitrification rates for ammonium in UK subsoils and aquifers. Ammonium attenuation was found to be highly sensitive to the clay mineralogy and pore size of the strata, the availability of oxygen and the chemical composition of the contaminated fluid. The values derived may have application in the initial (screening) phases of risk assessment where the conceptual model for the site under consideration matches that from which the presented data originate.