The low-temperature (500-1000 K) ignition of propane-air and ethane-air mixtures is studied in this work. An original detailed mechanism of 235 elementary reactions among 40 species, which models the ignition time of these fuels under a wide range of conditions for initial temperatures above about 1000 K, the so-called San Diego Mech, is revised and augmented to produce the two-stage ignition and negative-temperature-coefficient (NTC) behavior seen experimentally for these fuels below 1000 K. By using available kinetic data and introducing applicable steady-state approximations, it is shown that adding only four more reactions for each fuel to the original chemical mechanism succeeds in modeling the low-temperature regime. The predictions of this mechanism are compared with two types of experimental measurements, namely those from rapid-compression machines and those from static-reactor vessels, where NTC behavior and two-stage ignition are observed. The numerical results for the ignition delay exhibit reasonably good agreement with the experimental data from the rapid-compression machine, both qualitatively and quantitatively, as do the predictions for the static reactor, when radical loss by surface reactions is taken into account in an approximate manner. These results thus extend the range of applicability of the mechanism to lower temperatures that are of interest in various applications, such as HCCI engines. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.