The oxidation of carbon monoxide in equimolar mixtures (CO + O
2
) has been studied in a well-stirred open system (0.5 dm
3
) at vessel temperatures in the range 700-840 K, and reactant pressures up to 100 Torr (
ca
. 13.3 kPa) at a mean residence time of 8.5 s. Stationary states are established and oscillatory states sustained indefinitely in this system. The effect of small quantities of added hydrogen is studied by a carefully controlled, continuous supplement to the principal reactants. Four different modes of reaction (I-IV) have been characterized, and conditions for their occurrence mapped on a reactant pressure-vessel temperature (
p
-
T
a
) ignition diagram. Most boundaries are quite sharp, and some show evidence of hysteresis. Close to the axes, reaction is slow, non-luminous and non-oscillatory (I). Within a first broad promontory (II) reaction is accompanied by steady luminescence. Crossing the boundary is not accompanied by a step change in reaction rate, but there is a change in character from stable node (in I) to stable focus (in II). Auto-oscillatory luminescence occurs in a closed region (III) wholly within the promontory II. The effects of adding hydrogen on all these modes is to increase the reaction rates markedly and to make them non-isothermal; the boundaries between I, II and III are not as greatly affected. However, systems to which more than 0.10% H
2
have been added also display a new mode, of oscillatory ignition. This appears at first in a region (IV) of high temperatures and pressures but as more H
2
is increased its realm expands and it eventually dominates the ignition diagram, invading the region of luminescence and soon obliterating the oscillatory part completely.
