The kinetics of the nucleation and growth of Si films on amorphous SiO2-covered Si using rapid thermal chemical vapor deposition from SiH4 and Si2H6 (5% in He) were compared at temperatures between 600 and 800 degrees C and reactant gas pressures between 1 and 35 mTOrr. Quantitative assessment of the nucleation parameters and the structures of the deposited Si films have been determined using in situ real time single wavelength and spectroscopic ellipsometry. Tn addition to ellipsometry, atomic force microscopy. scanning electron microscopy, and cross-sectional transmission electron microscopy were : used ex situ to observe the nucleation stage and the microstructures of the films. This study compares the initial growth parameters for SiH4 : nuclei density (6 x 10(8) cm(-2)), nuclei size (94 nm), incubation time (4.2 min), and degree of selectivity (42 nm) with those for Si2H6 : 1.3 x 10(10) cm(-2), 31 nm, 0.4 min, and 10 nm, respectively, The incubation times for SiH4 and Si2H6 are different, as is the degree of selectivity, but they show similar activation energies of about 1 eV in the 600-800 degrees C range. The Si film quality in terms of surface roughness and grain structure was better for the Si film derived from Si2H6 than from SiH4.