The reactivity of the azidyl radical (N-3(.)) With olefins has been studied in acetonitrile solution using time-resolved UV-vis and infrared (TRIR) laser flash photolysis techniques. Azidyl radicals, formed by electron transfer from azide to excited state ketones, combine with excess azide in acetonitrile in an equilibrium reaction to form the pseudohalogen radical anion N-6(.-). The visible absorption of N-6(.-) (lambda(max) = 700 nm) is shown to be an ideal probe for the determination of absolute rate constants for the addition reactions of N-3(.) With a variety of alkenes (kadd) Bimolecular rate constants for the reactions of N-3(.) With a series of ring-substituted styrenes (p-CF3, m-CF3, p-Cl, H, m-CH3, p-CH3, p-CH3O) vary between 1 x 10(6) and 5 x 10(7) M(-1) s(-1). An excellent correlation of log(k(add)) With Hammett sigma(+) constants (rho(+) = -1.2, r = 0.991) is obtained illustrating the electrophilic nature of the azidyl radical. Rate constants for reaction with alpha- and beta-substituted styrenes and simple alkyl- and alkoxy-substituted olefins vary between 1 x 10(6) and 1 x 10(9) M(-1) s(-1) and correlate with the ionization potentials (a reflection of the relative HOMO energies) of the olefin. The addition reactions of N-3(.) were found to be dominated by polar effects with little contribution from steric factors. TRIR was utilized to follow the fate of azide and azidyl radicals by monitoring the bleach of N-3(-)(($) over bar v(max) = 2004 cm(-1)), the decay of N-6(.-) (($) over bar v(max) = 1842 cm(-1)); and the concurrent growth of beta-azidyl carbon-centered radicals at ca. 2100 cm(-1). The observation of the latter, which grow in with kinetics identical to the decay of N-6(.-), combined with the lack of evidence for competing hydrogen atom transfer in the reactions with alkyl olefins (even for 1,4-cyclohexadiene) confirms that reaction of N-3(.) with all olefins examined is by addition.