The rhenium–osmium (Re–Os) isotope system occupies a unique position amongst commonly used radiogenic parent–daughter isotope pairs on account of its siderophilic geochemical characteristics. Re and Os are also concentrated from seawater in organic-rich sediments and have been used to obtain absolute ages for the deposition of marine mudrock successions directly. Additionally, Re–Os analyses of ancient mudrocks have provided the best means so far of determining temporal changes in the Os-isotope composition of pre-Cenozoic seawater. The seawater Os-isotope record has yielded valuable information about several major episodes of past environmental change, and is particularly powerful when used in conjunction with other marine isotopic tracers such as carbon and strontium. The present contribution reviews some recent developments in the application of the Re–Os isotope system to geochronological and palaeoenvironmental problems in the Mesozoic and earlier. New results show that the Os-isotope composition of Callovian seawater (c. 160 Ma ago) was very unradiogenic (187Os/188Os ≈0.26) at the same time that the seawater Sr-isotope curve recorded its lowest value for the Phanerozoic, indicating that global continental weathering rates were very low at c. 10–15% of today's levels.