The quenching of electronically excited PH A (2) Sigma(+) radicals has been investigated in complexes of OH with molecular hydrogen, deuterium, and nitrogen and through complementary theoretical calculations. Many of the intermolecular vibrational levels supported by the OH A (2) Sigma(+) (v' = 0, 1) + H-2, D-2, and N-2 potentials have been characterized by laser-induced fluorescence and fluorescence depletion measurements of the complexes in the OH A (2) Sigma(+)-X (2) Pi 1-0 and 0-0 spectral regions. Homogeneous line broadening of the spectral features yields picosecond lifetimes for complexes prepared in levels derived from OH A (2) Sigma(+) (v' = 0) as a result of electronic quenching and/or chemical reaction. More extensive line broadening is observed for complexes excited to levels correlating with OH A (2) Sigma(+) (v' = 1). The corresponding decay rates are 10-75 times faster than obtained for upsilon' = 0 due to the opening of the vibrational predissociation channel and/or enhancement of the quenching/reaction processes upon OH vibrational excitation. Ab initio calculations of the OH (A (2) Sigma(+), X (2) Pi) + H-2 and N-2 potential energy surfaces reveal the minimum energy configurations, T-shaped O-H-H-2 and linear O-H-N=N, and the large increases in interaction energy upon electronic excitation of OH. The theoretical calculations also identify specific orientations, T-shaped H-O-H-2 and linear H-O-N=N, that lead to conical intersections between the ground-and excited-state surfaces and give rise to quenching of OH A (2) Sigma(+) by hydrogen and nitrogen.