The reaction of NO with CO on a catalyst of 0.5% Pt-3.4% MoO3 supported on a-Al2O3 was studied in different reactors at atm. pressure. A fixed-bed reactor with external recycle loop exhibited bistability and hysteresis as CO concn. or temp. were varied. In a tubular fixed-bed reactor non-isothermal periodic oscillations (DTmax = 40 K) were found in a specific pressure-concn.-temp. region (p-c-T region). The amplitude and period time of the oscillations changed smoothly inside the p-c-T region and died out suddenly at the borders of the region. The oscillations were of a relaxation type and lasted for more than 48 h. With fresh catalyst, a new quartz reactor and a specific pretreatment they were reproducible. It was shown that oscillations had kinetic origins and that the changes of the temp. synchronized the elementary oscillators. An autocatalytic model is proposed to explain the exptl. obsd. phenomena. [on SciFinder (R)]