A model system consisting of poly(tetramethyl bisphenol A polycarbonate) (TMPC) and poly(ethylene oxide) (PEO) was used to study the adhesion between miscible blends of polymers with very different glass transition temperatures. Contact mechanics experiments were performed where identical miscible blend layers were held into contact and separated at elevated temperatures. These results were correlated with measurements of PEO diffusion using dynamic secondary ion mass spectrometry. For low PEO concentrations in the blend, adhesion was obtained above a critical temperature that was consistent with the Fox equation, but substantial adhesion was not obtained at any temperature for blends with higher PEO contents. The results indicate that adhesion requires sufficient mobility of the high-T-g TMPC component and can be enhanced by small additions of PEO. Larger additions of the highly mobile PEO lubricate the interface, eliminating the enhancement in adhesion that would otherwise result from diffusion of the high-T-g component across the interface,