We have used X-ray photoelectron spectroscopy to study the chemical interactions at the interface formed during in situ deposition of Ti atoms on epoxy, triazine, and polystyrene surfaces. We find that for deposition on thick polymer films (1-2 mm) the primary component at the interface is TiO2 with small amounts of titanium nitride and titanium carbide. The source of the oxygen in the TiO2 is water or molecular oxygen dissolved in the polymer films. We also find that the Ti/triazine interface is more stable to heat treatment than the Ti/epoxy interface. This result is attributed to the higher glass transition temperature of triazine. For thin triazine films (similar to 100 Angstrom) we observe that titanium carbide is the dominant product, with smaller amounts of oxide and nitride. Aging in air causes the carbide and nitride to convert to the more thermodynamically-stable oxide. Titanium carbide is also the primary initial species at the Ti/polystyrene interface.