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 Ts. For all substrates, we found N∝κδ, with 0.8⩽δ⩽1.2, and the Ts 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