The chemistry of N2H4 on Si(100)2X1 and Si(111)7X7 has been studied using scanning tunneling microscopy (STM). On Si(100), the STM images show that the predominant pathway for adsorption is across the silicon dimers with the N-N bond parallel to the surface. On Si(111)7X7, the molecule behaves in a manner which is similar to NH3. That is, at low coverages the molecule adsorbs preferentially at center adatoms due to the greater reactivity of these sites, while at higher coverages it also reacts with the corner adatoms.