Protein oxidation can contribute to radiation-induced cell death by two mechanisms: (1) by reducing the fidelity of DNA repair, and (2) by decreasing cell viability directly. Previously, we explored the first mechanism by developing a mathematical model and applying it to data on Deinococcus radiodurans. Here we extend the model to both mechanisms, and analyze a recently published data set of protein carbonylation and cell survival in D. radiodurans and Escherichia coli exposed to gamma and ultraviolet radiation. Our results suggest that similar cell survival curves can be produced by very different mechanisms. For example, wild-type E. coli and DNA double-strand break (DSB) repair-deficient recA- D. radiodurans succumb to radiation doses of similar magnitude, but for different reasons: wild-type E. coli proteins are easily oxidized, causing cell death even at low levels of DNA damage, whereas proteins in recA- D. radiodurans are well protected from oxidation, but DSBs are not repaired correctly even when most proteins are intact. Radioresistant E. coli mutants survive higher radiation doses than the wild-type because of superior protection of cellular proteins from radiogenic oxidation. In contrast, wild-type D. radiodurans is much more radioresistant than the recA- mutant because of superior DSB repair, whereas protein protection in both strains is similar. With further development, the modeling approach presented here can also quantify the causes of radiation-induced cell death in other organisms. Enhanced understanding of these causes can stimulate research on novel radioprotection strategies.