A high-pressure stopped-flow study of the reaction of FeIII(aq) with HN3/N3- at 25 °C and ionic strength 1.0 mol kg-1 showed that two pathways predominated: (a) the previously unreported pathway involving Fe(H2O)50H2+ and N3- (rate constant kbb = 3.8 x 105 kg mol-1 s-1, volume of activation ΔVbb = +12.9 ± 1.5 cm3 mol-1) and (b) the reaction of Fe(H2O)5OH2+with HN3(fba = 5.0 X 103 kg mol-1 s-1.ΔVba = +6.8 ±0.5 cm3 mol-1). A third pathway involving Fe(H2O)63+ and HN3was barely detectable. The isomeric alternative to pathway b, in which Fe(H2O)63+ would react directly with N3-, can be ruled out by consideration of these kinetic data relative to those for reactions of FemIII with other nucleophiles, notably NCS-. In this context, the latter reaction has been reexamined by stopped-flow spectrophotometry at pressures up to 200 MPa to resolve serious discrepancies existing in the literature; for Fe(H2O)63+ + NCS-, ΔVab = -5.7 ± 0.3 cm3 mol-1 and the equilibrium molar volume change is +11.8 ± 0.3 cm3 mol-1, while for Fe(H20)5oH2+ + NCS-, ΔVbb = +9.0 ± 0.4 cm3 mol-1, agreeing with the stopped-flow results of Funahashi et al. (Inorg. Chem. 1983, 22, 2070) but contradicting the results obtained by several groups using temperature- or pressure-jump methods. Possible sources of these discrepancies are considered. The mechanisms of these reactions of Fe(H2O)5OH2+ may be described as Id, and those of Fe(H2O)63+ as Ia, within the limitations of this classification.