A radiochemical neutron activation analysis procedure is described for the determination of concentration levels of iridium in sedimentary rocks and in the geochemical standard PCC-1. After irradiation, the powdered rock samples and standard are dissolved with a mixture of HF, HNO3 and HClO4 in a teflon bomb. The final solution obtained, in dilute HCl, is passed through a column containing the cationic resin Bio Rad AG 50W-X8. The interfening radionuclides are sorbed by the resin. The effluent solution containing iridium is concentrated for counting by evaporation. Experiments with radioactive tracer for checking radiochemical separation yield are carried out. The accuracy of the method is evaluated by means of analysis of the USGS standard rock peridotite, PCC-1. The precision is about 25% in the ppb range. The method is used for the determination of iridium in 16 samples of sedimentary rocks collected at different depths in the Campos basin-RJ (BRAZIL).