The nucleation behavior during chemical vapor deposition (CVD) of silicon on Si(111)-(7X7) using disilane (Si2H6) is investigated with a scanning tunneling microscope. In the temperature range from 450 to 540 degrees C it is compared to molecular beam epitaxy (MBE). Whereas the latter is very well described within the framework of classical rate equation based nucleation theories in the complete condensation regime, the same approach fails for CVD growth. For MBE a critical nucleus i*=5-7 is found. Although the relationship between island density and growth rate in CVD also follows a power law as does MBE, the exponent is significantly higher than the theoretically expected value. The influence of hydrogen present on the surface during CVD growth is also discussed.