Concentrations of natural 129I and 36Cl in ground water from the Stripa granite, Sweden, were determined by accelerator mass spectrometry. The 129I values range from 1 × 10 6 to 2 × 10 8 atoms 1 −1, orders of magnitude greater than the estimated background concentration in pre-1945 rainwater of 2 × 10 4 atoms 1 −1. Expressed as a ratio 129I 127I , values range from 3 × 10 −12 to 2 × 10 −10, compared to about 1 × 10 −12 in pre-1945 rainwater. Ratios, as well as concentrations, generally increase with sample depth, which ranges from 360 to >1000 m below the surface. The accumulation of 129I in the water is attributed to subsurface production by spontaneous fission of 238U in micro-fractures in the granite matrix and subsequent transfer of this 129I by diffusion into fractures constituting the flow system of the granite. Uncertainties in the spontaneous-fission yield at mass 129 and in the assumption of steady-state mass flux prevent quantitative estimates of the rate of 129I transfer.