An in situ nanoscopic investigation of the prototypical surface alloying system Ag/Pt(111) is reported. The morphology and the structure of the ultrathin Ag–Pt film is studied using low energy electron microscopy during growth at about 800 K. A very rich dynamic behavior is uncovered in which stress relief plays a governing role. Initial growth leads to surface alloying with prolonged and retarded nucleation of adislands. Beyond 50% coverage dealloying proceeds, joined by partial segregation of Pt toward the center of large islands in violent processes. Subsequently, the growing islands coalesce and consequently irregularly shaped vacancy clusters evolve. These clusters are progressively filled by segregating Pt and assume a compact shape (black spots). As a result, at around 85% coverage, the strain of the initially pseudomorphological film is almost completely relieved and Pt segregation is at its maximum. Further deposition of Ag leads to transient reentrant alloying and recovery of the pseudomorphological layer. The black spots persist even in/on several-layer-thick films. Ex situ atomic force microscopy data confirm that these are constituted by Pt(-rich) structures. The (sub)monolayer films are strongly heterogeneous.