We employ a combination of high vacuum (chemical beam) epitaxy and in situ mass spectrometry to examine precursor surface decomposition chemistry during film growth of LiNbO3 and its constituent metal oxides on sapphire and Si (001). The four most common CVD precursors for LiNbO3 - [Li(OBut)](6), Li(tmhd), [Nb(OEt)(5)](2), and Nb(tmhd)(4) -are examined (OBut = tertiary butoxide, OEt = ethoxide, tmhd = 2,2,6,6-tetramethyl-3,5-heptanedionate). The metal diketonates are unstable at low pressures and temperatures, adversely impacting both storage and use. The metal alkoxides, while of greater thermodynamic stability, are subject to autocatalytic processes that can inhibit the film growth: these processes generate volatile metal-containing moieties that subsequently desorb from the surface. The degree of crystallinity of niobium oxide films grown from [Nb(OEt)(5)](2) depends on the presence of one of the autocatalytic elements, water. The growth rate of lithium niobate films depends strongly on the Li/Nb precursor ratio, suggesting a chemical interaction between the two precursors.