The transient behavior of the NO+CO reaction over a Pt catalyst during forced composition cycling has been investigated using an external recycle reactor. The convergence to a cycle-invariant state was very slow requiring as much as 90 hours of feed composition cycling. The effects of the feed composition, flow, temperature and the cycling frequency have been studied. The results can be partially explained by a mathematical model based on a seven-step reaction mechanism Slow convergence to cycle-invariance only occurred when the catalyst was subjected to an oxidizing pre-treatment.