The growth and stability of the Pb8×8 wetting layer on Si(111)7×7, which provides a foundation for growing quantum-size-effect nanocrystals, was investigated by in situ x-ray scattering. Our experimental results reveal that the wetting-layer structure evolves temporally over a remarkably broad range of temperatures and that there are two distinct temperature regimes of nonequilibrium behavior. When grown at lower temperature (below 170 ∘C), it was discovered that the wetting-layer structure changes with time, indicating that its disordered structure is not static; annealing in this regime improves the order of the wetting layer. Growth at higher temperature (170 ∘C < T < 250 ∘C), however, leads to a time-dependent degradation of the 8×8 structure due to the deterioration of the underlying Si(111)7×7. Thermal measurements determined an activation energy of 0.4 eV in the low-temperature regime, whereas in the high-temperature regime, a two-step process is observed, which has activation energies of approximately 1.3 and 1.9 eV. The results provide important considerations for understanding the anomalous kinetic behavior of quantum-size-effect Pb nanocrystals on Si(111)7×7, which is facilitated by the wetting layer.