We have investigated diamond deposition on porous silicon (PS) substrates without any pretreatment for nucleation enhancement. We find that the widely held expectation that, the highly rough nature of the PS surface would necessarily lead to a significant enhancement in the diamond nucleation density, does not hold. For the diamond deposition conditions used in this study, the nucleation density on PS (10(6) cm(-2)) is similar to that on untreated c-Si substrate. This indicates an absence of extra high-energy nucleation sites in spite of the micro-columnar surface of PS. As a result, the incubation time was often much longer on PS than on heavily scratched c-Si for the same diamond deposition conditions. Under such conditions, the XRD patterns of deposit on PS showed silicon carbide-related peaks in addition to weak diamond peaks. The results are compared with the earlier observations regarding diamond deposition on PS pretreated with diamond abrasive or negative DC bias. The comparative study, which needs further explanation, indicates a synergistic relation between the negative DC bias pretreatment and the effect of porosity of the substrate on diamond nucleation on PS substrate.