A high-resolution late Maastrichtian–early Danian seawater 87Sr/86Sr reference curve is constructed from two Cretaceous-Tertiary boundary (K-T boundary) sections: Bidart (France) and El Kef (Tunisia). The 87Sr/86Sr curve shows maxima at 0.3–0.4 Ma before the K-T boundary and at the K-T boundary. The first maximum could mark the onset of a major outflow of the Deccan Traps. The second maximum, a rapid 0.000 06 87Sr/86Sr, shift, extends from ∼3–4 m below to ∼1 m above the K-T boundary. This profile probably results from diagenetic smoothing of an originally sharp K-T boundary 87Sr/86Sr anomaly, rather than from a gradual process. The sharp shift could result from (1) the vaporization of the Chicxulub target rocks, (2) global wildfires, and (3) acid-rain leaching of soils and sialic surface rocks. Of these three possibilities, only Sr release by soil leaching combined with increased rainfall associated with the K-T event appears to be sufficiently large to produce the observed K-T 87Sr/86Sr anomaly.