The photochemistry of several para-substituted aryl azides was studied by laser flash photolysis (LFP) techniques in pentane. Para F, Cl, Br, I, CH3, CF3, COCH3, OCH3, and NO2 substituted aryl azides were studied. LFP of aryl azides generates singlet aryl nitrenes which were detected in absorption with maxima between 300 and 365 nm in pentane. The nitrene decay can be fit to an exponential function and analyzed to yield observed rate constant, k(obs), which was measured as a function of temperature. The value of k(obs) = k(R) + k(ISC) where k(R) and k(ISC) are the rate constants of nitrene rearrangement to azirine intermediates and intersystem crossing to the lower energy triplet states, respectively. Values of k(obs) decrease as the temperature is reduced and reach a limiting value attributed to k(ISC). Assuming that k(ISC) is temperature independent allows values of k(R) to be deduced along with the Arrhenius parameters to cyclization for para CH3, CF3, COCH3, F, Cl, and Br singlet phenyl nitrenes. The Arrhenius parameters to rearrangement closely resemble those of parent singlet phenyl nitrene with the exception of the para bromo compound. For para I and OCH3, k(ISC) is large and prevents measurement of k(R) The increased rate of intersystem crossing in these two singlet nitrenes is due to heavy-atom and to electron-donation effects, respectively. No transient absorptions were detected upon LFP of p-nitrophenyl azide.