The isostructural perovskite compounds PbFe1/2Ta1/2O3 and PbFe1/2Nb1/2O3 have been known for long time, and they are part of the important class of materials called multiferroic, where ferroelasticity, ferroelectricity, and ferromagnetism coexist. In the literature regarding PbFe1/2Ta1/2O3 and PbFe1/2Nb1/2O3, an "anomaly" of their low-temperature magnetic behavior has not always been reported. Moreover, both the origin of this behavior, and the cause for which it was not always observed, were never completely explained. In this paper, the magnetic behavior of the two compounds at low temperature has been extensively studied and explained as the occurring of a spin-glasslike transition.