Reliable ignition of reactant mixtures containing combustion products is important to the function of many practical combustion devices. Prior work has considered the influence of CO2 on ignition delay times and laminar flame speeds. This work examines the effects of dilution with CO2 on forced ignition and development of ignition kernels in CH4/air mixtures. Ignition kernels were initiated by a high voltage spark discharge in CH4/air/diluent mixtures with a bulk velocity of 5 m/s and with 0.7 <= Phi <= 0.8. Ignition probabilities and kernel growth rates were determined from radiation intensity measurements collected using a Telops MS-IR FAST infrared camera. Nitrogen was used as an inert in a subset of conditions to facilitate identifying changes in chemistry associated with dilution by CO2. Chemistry effects of diluents on ignition were isolated by examining undiluted, N-2 diluted, and CO2 diluted flows with equivalent adiabatic flame temperatures. Dilution with CO2 reduced the probability of ignition relative to undiluted and N-2 diluted conditions with the same adiabatic flame temperature. Ignition kernels diluted with CO2 also tended to be smaller than those initiated in both undiluted and N-2 diluted mixtures, with reduced growth rates. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.