Transient absorption, resonance Raman, and vibrational coherence spectroscopies are used to investigate the mechanisms of NO and O-2 binding to WT Tt H-NOX and its P115A mutant. Vibrational coherence spectra of the oxy complexes provide clear evidence for the enhancement of an iron-histidine mode near 217 cm(-1) following photoexcitation, which indicates that O-2 can be dissociated in these proteins. However, the quantum yield of O-2 photolysis is low, particularly in the wild type (less than or similar to 3%). Geminate recombination of O-2 and NO in both of these proteins is very fast (similar to 1.4 X 10(11) s(-1)) and highly efficient. We show that the distal heme pocket of the H-NOX system forms an efficient trap that limits the O-2 off-rate and determines the overall affinity. The distal pocket hydrogen bond, which appears to be stronger in the P115A mutant, may help retard the O-2 ligand from escaping into the solvent following either photoinduced or thermal dissociation. This, along with a strengthening of the Fe-O-2 bond that is correlated with the significant heme ruffing and saddling distortions, explains the unusually high O-2 affinity of WT Tt H-NOX and the even higher affinity found in the P115A mutant.