The electrical and optical properties of plasma polymer-metal composite films prepared by simultaneous plasma polymerization and metal evaporation depend on the plasma polymer properties and the metal volume part (filling factor). The microstructure of plasma-polymerized films with encapsulated silver particles was changed during thermal annealing up to 480 K. Transmission electron microscopy demonstrates the microstructural changes. The changes of the silver particle size and shape were analyzed by optical image processing. In one film with continuously varying filling factor, samples with conductivities above and below the percolation threshold were found after deposition. During the thermal annealing the d.c. conductivity of samples above the percolation threshold (f >f(c)) decreases from 10(-1) OMEGA-1 cm-1 to 10(-8) OMEGA-1 cm-1. In the UV-VIS-NIR region, the annealing affects the optical plasma resonance absorption of the metal particles. Relative intensity, half-width and spectral position of the transmission minima were changed. By consideration of the optical image processing results, the spectral shift of the plasma resonance was simulated with effective-medium theories.