A La(MnRh)O-3.15 catalyst, containing 1 wt% Rh, was prepared by a citrates/polyacrylamide gel method, leading to a sample with a specific surface area of 27 m(2)/g after calcination at 700 degrees C. This catalyst shows high activity in three-way catalysis reactions for the simultaneous reduction of NO and oxidation of CO and C3H6. However, the addition of 10 vol.% steam slightly deactivates the catalyst, mainly for NO reduction. Large amplitude oscillations of the feed streams between oxidizing and reducing compositions do not deactivate the sample : these variations in the composition are compensated for by the high mobility of the over-stoichiometric oxygen, which is available to oxidize CO under a reducing feed stream and is easily replenished by O-2 or NO in the presence of an oxidizing feed stream. This mobility corresponds to the reversible structural change, at low temperature, between hexagonal La(MnRh)O-3.15 and orthorhombic La(MnRh)(3.00), leading to an oxygen storage capacity of 0.15 mole [O] per mole of catalyst.