The adsorption of DNA bases-adenine (A), thymine (T), guanine (G), and cytosine (C)-and base pairs onto single-crystal n-CdSe substrates has been studied in several solvents, using the band gap photoluminescence (PL) of the semiconductor as a probe. In methanol solution, all four bases cause similar, reversible PL quenching, consistent with their acting as Lewis acids toward the surface. The PL changes can be well fit by a dead-layer model, indicating that adsorption increases the depletion width of the semiconductor by similar to 200-300 Angstrom, In DMSO solution, there is no PL response to individual bases. However, the complementary A-T and G-C base pairs yield a PL response, providing evidence that the surface can promote base pair formation. The A-T and C-G responses in DMSO correspond to depletion width increases of similar to 100 and 200 Angstrom and persist to similar to 45 and 75 degrees C, respectively. Competition experiments reveal a preference for C-G binding at elevated temperatures. In chloroform solution, the PL response of C-G base pairs can be distinguished from those of C and G individually, whereas A, T, and A-T are experimentally indistinguishable. Electronic and hydrogen-bonding effects that may contribute to the PL responses are discussed.