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Although it has long been known that metal-containing compounds can serve as catalysts for chemical vapor deposition (CVD) of films from other precursors, we show that metal-containing compounds can also inhibit CVD nucleation or growth. For two precursors A and B with growth onset temperatures TgA < TgB when used independently, it is possible that B can inhibit growth from A when the two precursors are coflowed onto a substrate at a temperature (T) where TgA < T < TgB. Here, we consider three precursors: AlH3⋅NMe3 (Tg = 130 °C, Me = CH3), Hf(BH4)4 (Tg = 170 °C), and AlMe3 (Tg = 300 °C). We find that (i) nucleation of Al from AlH3⋅NMe3 is inhibited by Hf(BH4)4 at 150 °C on two oxide surfaces (Si with native oxide and borosilicate glass), (ii) nucleation and growth of HfB2 is inhibited by AlMe3 at 250 °C on native oxide substrates and on HfB2 nuclei, and (iii) nucleation of Al from AlH3⋅NMe3 is inhibited by AlMe3 at 200 °C on native oxide substrates. Inhibition by Hf(BH4)4 is transient and persists only as long as its coflow is maintained; in contrast, AlMe3 inhibition of HfB2 growth is more permanent and continues after coflow is halted. As a result of nucleation inhibition, AlMe3 coflow enhances selectivity for HfB2 deposition on Au (growth) over Al2O3 (nongrowth) surfaces, and Hf(BH4)4 coflow makes it possible to deposit Al on Al nuclei and not on the surrounding oxide substrate. We propose the following criteria to identify candidate molecules for other precursor–inhibitor combinations: (i) the potential inhibitor should have a higher Tg than the desired film precursor, (ii) the potential inhibitor should be unreactive toward the desired film precursor, and (iii) at the desired growth temperature, the potential inhibitor should adsorb strongly enough to form a saturated monolayer on the intended nongrowth surface at accessible inhibitor pressures.