Previously we have shown how a single species X can be introduced, representing either HO 2 for high-temperature ignition or H 2 O 2 for low-temperature ignition, to develop an algorithm that covers the entire range of ignition, flame-propagation, and combustion conditions, without a significant degradation of accuracy, for hydrogen-air systems. By adding relevant CO chemistry to the hydrogen chemistry, this same approach can be applied to derive a comparably useful four-step reduced-chemistry description for syngas blends that have small enough concentrations of methane, other hydrocarbons , or other reactive species to be dominated by the elementary steps of the H 2 /CO system. The present communication reports the resulting extended algorithm. This work begins with the elementary steps of the detailed chemistry as listed in Table 1. We shall employ the numbering of the steps as given in the table, which identifies the 8 steps that are considered to be reversible and gives fitted parameters for the reverses of those steps.