The authors report real-time spectroscopic-polarimetric determinations of the initial phase of GaP heteroepitaxy by organometallic chemical vapor deposition on nanoscopically roughened (NR) (001)Si substrates, where polarimetry measurements are also used to quantify roughness. The authors compare the results with analogous data for GaP homoepitaxy and the initial phase of GaP heteroepitaxy on (001)GaAs. The large density of nucleation sites on NRSi significantly improves film continuity relative to nonroughened vicinal (001)Si substrates, but conditions that are typically used to grow GaP on (001)III-V surfaces generate metallic Ga, indicating that NRSi is more efficient at decomposing trimethylgallium than either GaP or GaAs. (C) 2007 American Vacuum Society.