We have compared the nucleation of pentacene on reduced and oxidized Si surfaces by a combination of x-ray reflectivity measurements and atomic force microscopy. For the reduced surface, the nucleation density is 0.007 $\mu {\mathrm{m}}^{-2}.$ Second monolayer (ML) formation starts at a coverage of Θ=0.6 ML, and the first layer is completely closed at a total coverage of 2 ML. For the oxidized surface, the nucleation density is larger by a factor of 100 (0.7 $\mu {\mathrm{m}}^{-2}).\mathrm{}$ Second ML formation also starts at Θ=0.6 ML, but the first layer closes already at 1.1 ML coverage, indicating nearly ideal layer-by-layer growth. For both terminations, the electron density obtained for the closed first monolayer is only 75% of the bulk value, indicating a reduced mass packing efficiency of the layer. Second ML islands are aligned relative to each other on an area limited by the lateral size of first ML islands, which act as templates for epitaxial growth.