Chemical effects of added CO2 on flame extinction characteristics are numerically studied in H-2/CO syngas diffusion flames diluted with CO2. The two representative syngas flames of 80% H-2 + 20% CO and 20% H-2 + 80% CO are inspected according to the composition of fuel mixture diluted with CO2 and global strain rate. Particular concerns are focused on impact of chemical effects of added CO2 on flame extinction characteristics through the comparison of the flame characteristics between well-burning flames far from extinction limit and flames at extinction. it is seen that chemical effects of added CO2 reduce critical CO2 mole fraction at flame extinction and thus extinguish the flame at higher flame temperature irrespective of global strain rate. This is attributed by the suppression of the reaction rate of the principal chain branching reaction through the augmented consumption of H-atom from the reaction CO2+H -> CO+OH. As a result the overall reaction rate decreases. These chemical effects of added CO2 are similar in both well-burning flames far from extinction limit and flames at extinction. There is a mismatching in the behaviors between critical CO2 mole fraction and maximum flame temperature at extinction. This anomalous phenomenon is also discussed in detail. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.