An integral part of a probabilistic analysis of the risk from fires in nuclear power plants is the estimation of the fire severity in compartments. The computer code COMPBRN implements physical models that predict the thermal hazards, e.g., temperatures and heat fluxes, during a compartment fire, as well as the failure time of objects, such as cables, that are subjected to these hazards. The COMPBRN II code is an improved version of COMPBRN; it includes radiative and convective heat losses from fuel elements and distinguishes between damage and ignition thresholds. Comparison with experimental results shows that the modifications generally lead to improved predictions.