[en] In the event radionuclides are released to the environment following an accident at a commercial nuclear power facility, measurement methods are required to assess the potential radiological hazards to the surrounding populace. This document reports the results of a study to evaluate the relative ability of selected portable radiological instruments to measure ¹³¹I in milk, on barren and grass covered ground, and in human and bovine thyroid phantoms. The instruments, sensitivities, accuracies, energy responses, and performance characteristics were evaluated in regard to operation at different temperatures and relative humidities and their resistance to mechanical shock and vibration