Iodine comes almost exclusively from organic matter in sedimentary basins and, therefore, can be used as a source indicator for hydrocarbons. The long-lived (half-life 15.7 m.y.) cosmogenic and fissiogenic isotope ¹²⁹I has been applied to tracing brine migration in the Anadarko basin, Oklahoma. Because all of the likely source formations for I are Paleozoic, the cosmogenic ¹²⁹I component has decayed to undetectable levels and all of the measured ¹²⁹I is of fissiogenic origin. Comparison of ¹²⁹I/¹²⁷I ratios measured in 12 brines from the platform area of the Anadarko basin with ratios determined for potential source rocks reveals that the Woodford Shale is the probable source formation for the extremely high I concentr tions observed in these brines. If the Woodford Shale is the exclusive source for I, the time of expulsion can be constrained to between 2 and 90 Ma. Although this date is much later than most estimates of expulsion time derived from conventional methods, a case is presented for more recent expulsion in the northern part of the basin, which was the sample area.