Measurements of (super 87) Sr/ (super 86) Sr ratios of planktonic foraminifers from paleomagnetically dated Deep Sea Drilling Project (DSDP) cores indicate that the (super 87) Sr/ (super 86) Sr ratio of seawater increased linearly between 6 and 4 Ma with a slope of 87 x 10 (super -6) /m.y. This suggests that (super 87) Sr/ (super 86) Sr variations in this time interval are useful for estimating ages in marine sequences. The theoretical stratigraphic resolution of the technique is 0.23 m.y., but the practical resolution is estimated to be 0.3-0.5 m.y.Strontium isotope ratios of five samples at the base of the Miocene/Pliocene (M/P) boundary stratotype at Capo Rossello, Sicily, have a mean (super 87) Sr/ (super 86) Sr values of 0.708995 + or -0.00002. Applying the regression equation derived from our (super 87) Sr/ (super 86) Sr vs. age data set for deep-sea sequences yields an age of 4.94 + or -0.50 Ma (95% confidence interval) for the M/P boundary stratotype, similar to ages estimated by biostratigraphic and magnetostratigraphic methods. Correlation of this mean (super 87) Sr/ (super 86) Sr ratio to the section at DSDP Site 519 suggests that the boundary is coincident with the onset of a major early Pliocene transgression, as recorded by a decrease in oxygen isotope values for benthic foraminifers. This study demonstrates the potential application of strontium isotope stratigraphy for correlating and dating problematic sequences, such as those deposited in restricted, shallow-water, or high-latitude environments.