This paper describes the development of a two-phase radiation model for the prediction of radiant heat attenuation in fires by water mist. It has been especially developed for incorporation in comprehensive field models of fire extinguishment by a water mist. The model predicts the opacity of the mist by summing over the absorption and scattering coefficients for each particle size present. A novel approach is proposed to reconcile the Eulerian form of the radiation transport equations with the Lagrangian particle tracking procedure. The model is fast, applicable to cases where the mist is modeled following the Lagrangian concept, and it retains in evidence the physics of the problem while avoiding complex mathematics. Validation with experimental measurements has shown that the model reproduces the water mist/radiation interaction with sufficient engineering accuracy. Results also indicate that radiation is a primary mechanism in fire extinguishment, since it contributes to nearly half of the total heat removal by the droplets.