Scanning tunneling microscopy shows that large two-dimensional Ag clus
ters on Ag(100) can diffuse. The value of the diffusion coefficient at
room temperature is of order 10(-17) cm(2) s(-1) and varies little, i
f at all, with cluster size in the range studied, 100 to 720 atoms per
cluster. This weak variation rules out periphery diffusion as the mai
n mechanism of cluster diffusion, suggesting instead two-dimensional e
vaporation-condensation. This conclusion is compatible with the energe
tics of atomic-scale events within the cluster and with the dissolutio
n of small clusters observed at low coverages.