Rate constants for the reactions N(D-2,P-2) + C2H2 and C2D2 have been measured using a technique of pulse radiolysis-resonance absorption between 220 and 293 K. Arrhenius parameters have been determined from the temperature dependence of the measured rate constants; the activation energies for the reactions of N(D-2) were about 0.5 kcal/mol, while those for N(P-2) were about 0.9 kcal/mol. The H/D isotope effect was found to be very small for both the N(D-2) + C2H2 and N(P-2) + C2H2 reactions. The rate constants for N(D-2) + C2H2 were found to be about 3 times as large as those for N(P-2) + C2H2. To understand the overall reaction mechanism of the N(D-2) + C2H2 reaction, ab initio molecular orbital calculations of the lowest doublet potential energy surface have been performed. It has been found that the initial step of the reaction is the addition of the N atom to the pi bond of acetylene. The rate constants have been calculated using conventional transition-state theory and compared to the experimental results. Possible reaction pathways are discussed on the basis of the ab initio results.