Under continuous flow conditions the oscillatory NO + CO reaction on Pt(100) exhibits bistability in the temperature range below 440 K. In this range, an inactive stationary branch coexists with a reactive oscillatory branch. The transition from the inactive branch, which is only metastable, to the truly stable active branch proceeds via propagating reaction fronts. The associated nucleation process has been investigated in the 10-6 mbar range using photoemission electron microscopy (PEEM) as a spatially resolving method. A Pt sample of cylindrical shape whose surface comprises the highly active orientational range between (100) and (3 10) was used as a catalyst. Depending on the degree of supersaturation, both heterogeneous and homogeneous nucleation have been found. Homogeneous nucleation manifests itself in the occurrence of a so-called “surface explosion” at Tc = 440 K. Below this temperature, heterogeneous nucleation occurs. Depending on the preparation of the inactive state, we observe two different nucleation scenarios which we can tentatively assign to two different kinds of defects.