Tropospheric chemical mechanisms can generate oscillations in which the concentrations of all reactive chemical species oscillate on timescales ranging from days to years, depending on certain parameter ranges of specified sources for NOx and a reduced species (e.g., carbon monoxide, isoprene, or methane). A simple carbon monoxide oxidation mechanism is presented that exhibits oscillatory solutions qualitatively similar to those found in more complex mechanisms. This allows the derivation of a two-variable oscillatory model for ozone and carbon monoxide. Using this model, an analytic estimation of the oscillation period is given, the chemical sources required for oscillatory solutions are evaluated, and the mechanism for the oscillation is elucidated. Some of these results can be extended to more complex oscillatory mechanisms.¿ 1997 American Geophysical Union