Abstract From the critical interpretation of many examples of rubidium-strontium (Rb-Sr) dating of shales and related rocks, two main conclusions can be drawn: (1) Isochron diagrams of whole rock samples of pelitic sediments, originated from the same depositional environment, in many cases allow the determination of the age of sedimentation. In such cases, a uniform dispersion of clastic material within the basin must be inferred. (2) Isochron diagrams for subsystems of the fine fraction (insoluble and soluble materials) can indicate almost always the time of the diagenesis, anchi- or epi-metamorphism suffered by each sample. In such cases, Sr isotopic homogenization among clay minerals and soluble material occur. Several examples for the above referred concepts were found in the literature, and others were produced in the Sao Paulo Geochronology Laboratory on more than 300 samples from the following sedimentary units: Trombetas Formation (Silurian, marine); Botucatu Formation (Jurassic, continental); Rio do Rasto Formation (Permian, continental); Irati Formation (Permian, marine); Rio Bonito Formation (Permian, marine); Sepotuba Formation (Cambrian, marine); Estancia Formation (Cambro-Ordovician, marine); and Bambui Group (late Precambrian, marine).