Microstructures of alkali- and/or heat-treated films on a Ti-15Zr-4Nb-4Ta alloy were analyzed by means of scanning electron microscopy, thin film X-ray diffraction and Auger electron spectroscopy. The cohesiveness of films was also evaluated by scratch tests. The films were formed by immersion in 5M aqueous NaOH solution at 60 °C for 86.4 ks (alkali treatment) followed by heating at 400–600 °C for 3.6 ks. The film on alloy formed by alkali treatment exhibits the same strucutre as that formed on an alkali-treated titanium. Compositional gradient of alloying elements, Zr, Nb and Ta, is detected in the film. The cohesion of alkali-treated film is considerably increased by the heat treatment, and the maximum cohesion is obtained by heating at 600 °C. The increase in cohesion of alkali-treated film by heat treatment is due to both the diffusion of Zr into film and the formation of sodium titanate on substrate.