The stable silicon isotope composition (δ30Si) of sponge spicules and radiolaria can be used as a proxy for past changes in dissolved silicon (Si) concentrations and Si cycling processes. Sponge spicules and radiolaria were analyzed for their δ30Si signatures from DSDP Site 208 in the Southwest Pacific. These data are in association with a study aiming to expand our knowledge about changes in the marine Si cycle across the Cretaceous - Paleogene boundary. Our δ30Si data (for opal-A) range from -2.7‰ to -1.5‰ for the sponge record (65.5 Ma and 64 - 60 Ma) and from +0.1‰ to +1.2‰ for the radiolarian record (70 Ma, 65.5 Ma and 64 - 60 Ma).