Modifications in the mbidium-strontium-strontium isotope system during low-temperature, geologic processes have been determined for major lithic units. Rubidium-strontium ratios in basalt and granite increase markedly during weathering, but strontium isotopic composition is not altered significantly in many profiles. These relations may be useful in distinguishing active and relict weathering profiles. Deposition of aluminosilicate detritus in the marine environment may result in a further increase of rubidium-strontium ratios for the detritus, owing to rubidium fixation. Strontium in even the finest fraction of aluminosilicates deposited in the ocean, however, apparently does not equilibrate with marine strontium, even after prolonged contact with sea water and interstitial marine water. Thus the isotopic composition of strontium in deep-sea sediments may be used as an indicator of geologic provenance. Diagenesis of aluminosilicates does not generally result in equilibration of strontium isotopes, except possibly after the sediment has been deeply buried. This lack of strontium isotope equilibration during low-temperature processes places constraints on the dating of sediments and sedimentary rocks by the rubidium-strontium method.