Threshold voltage (V_th) behavior of nitride read-only memories (NROMs) was studied after irradiation with photons (γ- and X-rays), light and heavy ions. Both programmed and nonprogrammed single cells were investigated. The data suggest that two main physical phenomena are contributing to V_th variation and that the V_th loss and the variability can be modeled by a Weibull statistics with a shape parameter k ~ 2.2 regardless of the irradiation species and total dose. The same peculiarities were found in large memory arrays, confirming the results from single-cell studies but with significantly larger statistics. Hence, once the irradiation dose is known, the V_th loss distribution can be obtained, thus providing a predictive model of the radiation tolerance of NROM memory arrays.