The use of the 187 Os/ 186 Os ratio as a diagnostic indicator of cosmic osmium in high osmium (and iridium) layers in sedimentary strata, such as those found at the Cretaceous/Tertiary boundary, is explored. The 187 Re/ 186 Os ratio in meteorites is 3.2 and in crustal rocks it is 400, resulting in different 187 Os/ 186 Os ratios in the two systems after billions of years have passed. (The radioactive decay constant of 187 Re is 1.6 × 10 −11 y −1 .) Meteorites now have a 187 Os/ 186 Os ratio of about one, and the earth’s crust averages (assuming no crustal recycling through the mantle) between 13 and 30. As a result, Os from the crust concentrated in certain sedimentary strata should be readily distinguished from Os from extraterrestrial projectiles.