Experimental and numerical studies have been performed on the thermal and chemical effects of buffer gas composition on low temperature ignition of iso-octane and n-heptane. Experiments were conducted using a recently developed rapid compression machine in the temperature range of 600-850 K. Three buffer gases were studied including nitrogen (N-2), argon (Ar), and a mixture of Ar and carbon dioxide (CO2) at a mole ratio of 65.1%/34.9%. Iso-octane was studied at 20 bar, phi = 1, and a dilution level of buffer gas to O-2 of 3.76:1 (mole ratio). n-Heptane was studied at 9 bar, phi = 1, and a dilution level of buffer gas to O-2 of 5.63:1 (mole ratio). For experiments where two-stage ignition was observed, the buffer gas composition had no impact on the first-stage ignition time but, as expected, it caused differences in the total heat release, pressure and temperature rise after the first-stage ignition. As a consequence, significant differences were observed for the total ignition delay time as a function of the buffer gas composition, with up to 40% and 42.5% faster total ignition time for iso-octane and n-heptane, respectively, by using Ar instead of N-2. The chemical effects of the buffer gas composition were studied experimentally by comparing the results of the N-2 and Ar/CO2 (65.1%/34.9%) mixtures, recognizing that while the Ar/CO2 mixture has the same heat capacity as N-2, its predicted combined third-body collision efficiency is about 76% higher than N-2. The experimental results showed negligible chemical effects on the first-stage and total ignition delay times. Numerical simulations were carried out over a wider range of temperatures for pure N-2, Ar, and CO2 as buffer gases. Results showed that thermal effects are very pronounced and dominated at the negative temperature coefficient and two-stage ignition conditions, which is consistent with the experimental results and previous studies in the literature. However, the simulation results also showed at temperatures higher than 850 K, the chemical effects of CO2 became more important than the thermal effects. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.