The total quenching rate of NCl(a1Δ) metastables by ground-state I(2P3/2) atoms over the temperature range 297−473 K has been studied in a flow reactor. The room-temperature rate coefficient is (2.1 ± 0.4) × 10-11 cm3/(molecule s). In this temperature range, the temperature dependence for the total quenching rate is described by an Arrhenius expression of k(T) = 1.1 × 10-10 exp(−519 ± 143 K/T) cm3/(molecule s). The rate coefficient for the energy transfer of NCl(a1Δ) to I*(2P1/2) at 300 K is (1.5 ± 0.7) × 10-11 cm3/(molecule s), which is in good agreement with a previous measurement. The magnitude of the energy transfer rate implies a large fraction of the I(2P3/2) is converted to I*(2P1/2), and the NCl(a1Δ) + I(2P3/2) reaction is an efficient source of I*(2P1/2). Under the current experimental conditions, a simple kinetic model that simulated the experimental time histories of NF(a1Δ) and NCl(a1Δ) infers the Cl + N3 rate coefficient of (1.6 ± 0.4) × 10-11 cm3/(molecules s) and is slower than previously measured.