The NOel temperatures (TN) of synthetic goethites (c•-FeOOH), substituted with diamagnetic elements (A1, Ga) have been determined from the temperature dependence of their low-field susceptibility using a Kappabrige AC bridge. These temperatures proved to be identical to those derived fi'om more conventional measurements of high-field or remanent magnetization versus temperature, and in good agreement with those inferred fi'om M0ssbauer models. The low-field susceptibility based-technique appears to offer a precise and rapid method of determining T N. While NOel temperature coincides with maximtan unblocking temperature in Ga-goethites, supporting the assumption that antiferromagnetism and parasitic ferromagnetism have a similar origin in goethite, the lack of clear correlation between saturation remanent magnetization and increasing diamagnetic substitution suggests that the weak ferromagnetism of goethite is due to substitution independent defects induced during crystal growth.