We present here a study on the surface plasmon resonance (SPR) in Au films deposited onto glass substrates and annealed in air at different temperatures. The initial Au films exhibit the resonant absorption of extended surface plasmons which depends on the film thickness. Thermal treatments promote the modification of the continuous films toward the formation of Au isolated islands. The morphological features of the islands depend on the film initial thickness and annealing temperature. The optical properties of the films are qualitatively modified as a consequence of the morphological changes. For films with initial thickness below 30 nm, the islands exhibit localized SPR while thicker films lead to islands large enough to hold extended SPR.