The extinguishment characteristics of CO2 as a fire-suppressing agent have been studied experimentally and numerically using a methane-air laminar co-flow diffusion flame stabilized on a cup burner. Direct numerical simulations of cup-burner flames under various gravitational forces were performed using a time-dependent, axisymmetric mathematical model with a detailed-chemical-kinetic mechanism for CH4/O-2 combustion. Experiments with cup-burner flames under normal-gravity (1g) conditions were performed for comparison purposes. Both the computed flicker frequency and the predicted critical concentration of CO2 for extinguishing the flame compared well with the respective quantities measured in the experiments. As the buoyancy force is reduced, the flicker frequency decreases, the flame diameter increases, the tip opens, and the base becomes vertical. It is predicted that the cup-burner flame ceases to flicker for gravitational forces corresponding to less than 0.5g. Numerical experiments revealed that radiative heat loss is predominantly responsible for flame quenching (opening) in the tip region under microgravity (0g) conditions. In contrast, 1g flames are affected only slightly by the radiative heat loss. Calculations are made by adding different amounts of CO2 to the air stream for obtaining the critical volume fraction of CO2 to extinguish 0g flames. The behavior is similar to that observed in 1g flames: the addition of CO2 destabilizes the flame base, which then moves downstream in search of a new stabilization location. For CO2 volume fractions greater than 19.1%, the flame base moves out of the computational area, as it cannot find a stabilization point within the domain. This critical concentration for the 0g flames is similar to 32% higher than that computed for the same flames under 1 g conditions. Calculations made by ignoring radiation for the limiting flame under 0g conditions yielded a stable flame. This study suggests that it is important to consider radiation heat losses when estimating the extinguishment limits of cup-burner flames in microgravity. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.