Kinetics of phosphorus accumulation on grain boundaries (GB) in iron-based alloys is treated theoretically, taking into account both the radiation-induced segregation (RIS) in the matrix and the Gibbsian adsorption (GA) at GB. For steady-state conditions analytical expressions are derived for component profiles near GB and component concentrations on GB. Modeling of phosphorus accumulation at GB in iron alloys is carried out using two different models: 1) the McLean's model generalized to take into account the radiation-enhanced phosphorus diffusion via vacancy and interstitial mechanisms as well as RIS near GB, 2) a model of RIS in a ternary Fe-P-Ni alloy accounting for the binding energy of phosphorus atoms with interstitials as well as a possibility of high phosphorus content near GB at high irradiation doses. Predictions of modeling are analyzed regarding the dependence on temperature, dose and Fe and P diffusion parameters available for iron alloys. It is shown that the GB phosphorus concentration calculated as a function of temperature reveals one or two maxima depending on dose and the choice of material parameters.