We present a comparative study of the nucleation and growth of pentacene monolayer islands in the submonolayer regime onto inorganic substrates of SiO₂ and sapphire (Al₂O₃) and organic substrates of poly(methyl-metacrylate) (PMMA). We have determined the scaling laws that govern the saturated nucleation density per unit area N as a function of two essential deposition parameters: the deposition rate κ and the substrate temperature T_s. For all substrates, we found N∝κ^δ, with 0.8⩽δ⩽1.2, and the T_s dependence of the nucleation density follows a typical activated Arrhenius law. Whereas a critical nucleus size of i=2 is obtained for all investigated substrates, the activation energy for nucleation depends significantly on the dielectric. The difference in activation energy of nucleation on PMMA and SiO₂ is due to different molecule-substrate interactions