We have determined the gas-phase interstellar abundance of the dominant ions of tin and cadmium (Sn II and Cd II) in the diffuse clouds toward 14 and 5 stars, respectively. Our measurements show that tin is exchanged between the gas and dust phases of the diffuse interstellar medium, while cadmium is not. The sight lines showing the highest gas-phase tin-to-hydrogen ratios (the least amount of Sn depletion) have values of Sn/H that are supersolar. Tin is the first element to show a well-determined interstellar gas-phase abundance that appears to be enriched with respect to the Sun. This finding is direct evidence of s-process enrichment of the ISM by low- to intermediate-mass asymptotic giant branch stars. The average Cd/H abundance ratio in our sample is 0.09 dex below the Sun's. This ratio does not preclude an enhancement of cadmium with respect to the Sun if Cd is incorporated into dust and/or if the adopted f-value for the cadmium transition has a large error.