Gas-phase adsorption of several boranes (BF3, MeBBr2, Me2BBr, and Et3B) onto the surface of etched, single-crystal n-CdSe quenches the band gap photoluminescence (PL) relative to its intensity in vacuum (pressure approximately 10(-5) Torr). The PL quenching is consistent with the Lewis acidic nature of the boranes and is dependent on substituents: PL quenching follows the order BF3 > MeBBr2 > Me2BBr > Et3B. The magnitude of the PL quenching can be fit to a dead-layer model, permitting an estimate of the adduct-induced expansion of the depletion width in the semiconductor resulting from exposure to these Lewis acids; dead-layer expansions as large as approximately 600 angstrom have been measured. The borane-induced PL changes are pressure-dependent and, for all but BF3, can be fit by a simple Langmuir adsorption isotherm model, yielding equilibrium constants for adduct formation, K(P), that range from on the order of 10 atm-1 for Me2BBr and Et3B to 10(3) atm-1 for MeBBr2. Detection limits can reach values of as low as approximately 0.001 Torr for BF3. The use of these effects as the basis for on-line sensors in chemical vapor deposition (CVD) processes is discussed.