The radiotracer technique was used to assess the influences of sediment-metal contact time or aging (up to two years) on the bioavailability and geochemical speciation of Cd and Zn in sediments. Bioavailability was quantified by measuring the assimilation efficiency of metals in two deposit-feeding invertebrates (sipunculans and soldier crabs) and the extraction by the sipunculans' gut juices. Sediment aging generally did not significantly affect the Cd speciation in the sediments, Cd assimilation, and Cd extraction. In contrast, sediment aging significantly affected the Zn distribution in different geochemical phases and the Zn bioavailability. With increasing aging, the Zn distributed in the carbonate phase decreased, whereas that in the reducible phase increased. Accordingly, the Zn gut-juice extraction decreased significantly. Two years of aging were not sufficient for Zn to be associated with the organic and residual phases. A significant positive correlation was found between Cd gut-juice extraction and assimilation efficiency. Gut-juice extraction of Cd and Zn generally increased with metal distribution in the exchangeable and carbonate phases but decreased with that in the reducible phase. Our results suggest that different metals are influenced by sediment aging differently and that geochemical speciation analysis is useful in studying the bioavailability of sediment-bound metals. This study may have implications for designing sediment toxicity tests using spiking techniques and for understanding the fates of anthropogenically derived metals in sediments.