The mechanisms of chemically-induced nitritation in biofilm systems were explored using a 1-D model with hydroxylamine (NH2OH) as a case study. Replacing conventional nitrification in wastewater treatment processes with nitritation and anammox would reduce both O2 demand and the need for carbon addition. Implementing nitritation in a biofilm system would have further benefits due to the high cell density, high solids retention time, and the presence of microenvironments that foster a wider variety of transformations. This research used modeling to explore the potential of NH2OH-induced nitritation in co- and counter-diffusional biofilms under mainstream conditions with NH2OH supplied either from the bulk liquid or the base of the biofilm. The results suggested that achieving full nitritation was possible in both co- and counter-diffusional biofilms. For co-diffusional biofilms, both supply methods (bulk liquid or the base of the biofilm) were effective at inducing nitritation with low concentrations of NH2OH. However, the retention of a small amount of NOB even at high NH2OH concentrations suggests that nitritation could be more easily compromised. For counter-diffusional biofilms, NH2OH supplied from the bulk was unable to induce nitritation, but when supplied from the base it was able to induce nitritation and fully eliminate NOB, even at low NH2OH concentrations.