Carbon isotope chemostratigraphy is a powerful tool in the study of non-fossiliferous metasedimentary carbonate rocks. This study deals with d13C fluctuation within the Precambrian Jucurutu and Seridó Formations, Borborema province, in northeastern Brazil whose age has been a matter of debate for over three decades. While some researchers have assumed a Paleoproterozoic age for both Formations, radiometric ages have raised the possibility that they are Neoproterozoic. In an attempt to contribute to this debate, a detailed carbon isotope chemostratigraphy of these two Formations have been undertaken. The Jucururu Formation displays a remarkable homogeneous composition of δ13C with values varying from +8.5 to +10%oPDB. The Seridó Formation, by its turn, shows a more pronounced variation with, values varying from +6.5 to +10.5 %oPDB. According to our carbon isotope data versus the available δ13C secular variation curves, these two Formations could be either Paleoproterozoic (2.0-2.15Ga) or Neoproterozoic (0.6 to 0.65 Ga). Further Sr and Nd isotope studies of these carbonate sequences, coupled with more detailed field geology, are necessary to test the Neoproterozoic-age hypothesisCarbon isotope chemostratigraphy is a powerful tool in the study of non-fossiliferous metasedimentary carbonate rocks. This study deals with d13C fluctuation within the Precambrian Jucurutu and Seridó Formations, Borborema province, in northeastern Brazil whose age has been a matter of debate for over three decades. While some researchers have assumed a Paleoproterozoic age for both Formations, radiometric ages have raised the possibility that they are Neoproterozoic. In an attempt to contribute to this debate, a detailed carbon isotope chemostratigraphy of these two Formations have been undertaken. The Jucururu Formation displays a remarkable homogeneous composition of δ13C with values varying from +8.5 to +10%oPDB. The Seridó Formation, by its turn, shows a more pronounced variation with, values varying from +6.5 to +10.5 %oPDB. According to our carbon isotope data versus the available δ13C secular variation curves, these two Formations could be either Paleoproterozoic (2.0-2.15Ga) or Neoproterozoic (0.6 to 0.65 Ga). Further Sr and Nd isotope studies of these carbonate sequences, coupled with more detailed field geology, are necessary to test the Neoproterozoic-age hypothesis.