A mechanistically based nitrification model was formulated to facilitate determination of both NH4+-N to NO2--N and NO2--N to NO3--N oxidation kinetics from a single NH4+-N to NO3--N batch-oxidation profile by explicitly considering the kinetics of each oxidation step. The developed model incorporated a novel convention for expressing the concentrations of nitrogen species in terms of their nitrogenous oxygen demand (NOD), Stoichiometric coefficients relating nitrogen removal. oxygen uptake, and biomass synthesis were derived from an electron-balanced equation. A parameter identifiability analysis of the developed two-step model revealed a decrease in correlation and an increase in the precision of the kinetic parameter estimates when NO2--N oxidation kinetics became increasingly rate-limiting. These findings demonstrate that two-step models describe nitrification kinetics adequately only when NH4+-N to NO3--N oxidation profiles contain sufficient information pertaining to both nitrification steps. Thus, the rate-determining step in overall nitrification must be identified before applying conventionally used models to describe batch nitrification respirograms.