In order to examine the cause of the reactor fuel pin pellet-cladding interaction phenomenon (PCI), stress corrosion cracking (SCC) experiments of Zircaloy under iodine, iron iodide, aluminum iodide, cesium iodide, and cadmium were undertaken. The rupture lifetime was measured as a function of stress, temperature, and the equivalent pressure of the corrosive agents. Tests with CsI and simultaneous irradiation were also performed to check whether radiation decomposition can induce Zircaloy SCC. Iodine, iron iodide, and aluminum iodide substantially reduced the failure times in comparison to those of control specimens at the same stress and temperature. A critical stress of ~379 MPa for iodine and iron iodide separates burst type failure from low-stress short-slit failures. Both types showed brittle cleavage fracture surfaces. The presence of cesium iodide did not have any influence on the failure time of Zircaloy. The failure was burst-type, and the fractography was ductile. Simultaneous radiation tests with cesium iodide did not cause reduction in failure time either. All specimens failed under cadmium vapor by a burst mode, but the fractography showed cleavage brittle characteristics and the failure times were much smaller than those of control specimens.