Cathodic charging of purified iron with hydrogen at room temperature and in the absence of an external stress results in the formation of intergranular and/or transgranular cracks for all purities of iron examined. The transgranular cracks are crystallographic, forming on variants of {110} or possibly {112} planes. The crack path is a sensitive function of heat treating temperature, cooling rate, and interstitial content; the path can be changed systematically and reversibly from entirely transgranular to entirely intergranular by suitable control of these parameters. This change is similar to changes in the low-temperature fracture path of purified iron tested in tension, suggesting that the two effects are controlled by the same mechanism. Although this mechanism is undoubtedly related to the relative amounts and partitioning of solutes between grain boundaries and grain interiors, no detailed explanation is advanced. It is suggested that the answer will have to be found in the details of the interactions between carbon, oxygen, hydrogen, and possibly vacancies.