Infrared reflection absorption spectroscopy has been used to study the adsorption and coadsorption of Ga-(C2H5)(3) (TEG), NH3, and N2H4 at 120-160 K on MgO films grown epitaxially on Mo(100). All three species adsorb molecularly and, upon warming, desorb without leaving any residue detectable by Auger spectroscopy. At submonolayer coverage, NH3 and N2H4 adsorb in well-defined geometries. NH3 adsorbs via an Mg-N bond normal to the surface, and N2H4 is suggested to adsorb with one NH2 forming an Mg-N bond and the other H-bonded to an oxide ion. Evidence is also found for relatively strong TEG-MgO bonding. Exposure of preadsorbed TEG to NH3 yields a TEG-NH3 adduct, whereas the reverse sequence does not displace NH3 from the MgO surface. Preadsorbed N2H4 bonded to the MgO surface reacts immediately upon exposure to TEG. In contrast, preadsorbed TEG bonded to MgO does not react with N2H4 but, instead, inhibits crystallization of the N2H4 overlayer. The interaction within condensed, coadsorbed multilayers of TEG and N2H4 is more complicated but, with mild annealing, ends in reaction, most likely the formation of an adduct.