A value of the eddy diffusion coefficient K of ≈1.5¿106 cm2 s-1 in the middle atmosphere of Mars was obtained from Phobos 2 solar occultation measurements of dust, ozone, and clouds at low latitude. The aim of the present study is to complete this picture by using a steady state photochemical one-dimensional model. The main regulation mechanism on O2 is the reaction of O with itself, whose rate depends on the value of K in the middle atmosphere. By comparing calculated and observed values of the O2 abundance, an upper limit of ≈2¿107 cm-2 s-1 of K is inferred. By including an additional constraint provided by H2 balance, a lower limit ≈4¿106 cm-2 s-1 may be placed. It results from the present analysis that the most realistic value of K to be used in works resorting to one-dimensional modeling (long-term evolution, escape, surface/atmosphere exchanges) is ≈107 cm2 s-1 rather than ≈106 cm2 s-1. The difference between theroretical and observational values might be due to the regular occurrence of global dust storms, whose effect should be to increase the yearly value of K. The present study suggests less than 3 precipitable micrometer (pr-μm) of the yearly averaged water vapor column, unless H2O is confined in a layer near the ground. Although the first possibility seems more probable, the second hypothesis cannot be ruled out. It could reflect a continuous supply of H2O from the regolith to the atmosphere on a seasonal scale. The loss to production ratio of CO, which has a lifetime of ≈5 years, is shown to depart from unity by no more than ≈10% over a wide range of atmospheric conditions. The stability of the Martian atmosphere is therefore realized in the classical frame of homogeneous chemistry. ¿American Geophysical Union 1995