The temperature dependence of the fluorescence of trace levels of pyrene was employed to measure the glass transition temperatures (T(g)s) of polystyrene (PS) domains within ultrathin films of poly(styrene-block-methyl methacrylate) (P(S-b-MMA)) and poly(styrene-block-2-vinylpyridine) (P(S-b-2VP)) diblock copolymers supported on silica after self-assembly into lamellar morphologies. The PS domains within the two block copolymers exhibited dramatically different T-g-nanoconfinement behavior. The T(g)s of the PS domains within the P(S-b-MMA) films were observed to decrease with decreasing film thickness in accordance with T-g reductions observed for single-layer PS homopolymer films; in contrast, the T-g of the PS domains within the P(S-b-2VP) films exhibited no change within experimental uncertainty upon confinement. The T(g)s of the block copolymer films showed excellent agreement with those of multilayer films constructed from homopolymer layers with an equivalent morphology, indicating that chain connectivity across the immiscible interface has little impact on cooperative segmental dynamics within the individual layers/domains. The difference in the T-g-nanoconfinement behavior exhibited by the PS free-surface layer of P(S-b-MMA) and P(S-b-2VP) block copolymer films, and equivalently exhibited by the PS free-surface layer of PS/PMMA and PS/P2VP bilayer films, is due to the much stronger attractive interaction at the P2VP-silica substrate interface. Once the P2VP-silica substrate interaction is removed, the P2VP underlayer is no longer able to suppress the enhanced mobility of a PS free-surface layer when the underlayer thickness is made sufficiently thin (