β-solidifying Ti-43.5Al-4Nb-1Mo-0.5B has attracted considerable attention owing to its higher strength and excellent creep resistance at elevated temperature. Indeed, its application temperature is much higher than that of Ti-48Al-2Cr-2Nb. Because γ-TiAl alloys are exposed to air at elevated temperatures for a long time during application, an oxidation layer is formed in the surface. The oxidation layer, which is potentially harmful to the mechanical properties of the crack nucleation sites, was observed near the surface. Concerning the β-solidifying Ti-43.5Al-4Nb-1Mo-0.5B, it has median Nb content and low Al content. Additionally, a considerable β₀-phase with lower Al content is retained. To better understand the influence of the composition and microstructure on the oxidation behavior of γ-TiAl alloys, it is necessary to investigate the oxidation behavior and microstructure evolution in the surface of β-solidifying γ-TiAl alloys during thermal exposure. In this study, samples of β-solidifying Ti-43.5Al-4Nb-1Mo-0.5B were obtained by investment casting and thermal exposure at 700°C for different times, and the oxidation behavior and microstructure of different phases in the surface were compared. The results showed that the constituents of the oxidation layer on the surface varied with the exposure time. The volume fractions of TiO₂-R, α-Al₂O₃, Ti₂AlN, and Nb₂Al increased by increasing the exposure time. Metastable κ-Al₂O₃ was detected in the sample exposed for a short time, but it was transformed into α-Al₂O₃ after exposure for 200 h. Moreover, metastable Ti₄O₇ and TiAl₂O₅ were detected in samples exposed for 200 and 500 h. The microstructures, morphologies, and heights of oxidations in the surface of a specific phase are different, varying by increasing the exposure time. These variations are related to the different oxidation behaviors during thermal exposure, i.e., the γ-phase experienced selective oxidation after a short time exposure, α₂-phase changed from internal oxidation to selective oxidation when the exposure time reached 200 h, while the β₀-phase suffered internal oxidation during the entire exposure. The different oxidation behaviors of each specific phase contributed to the different Al contents. Dispersed TiO₂ was formed during internal oxidation, and it kept growing during thermal exposure, forming a continual layer at the end. The continual Al₂O₃ layer was formed during selective oxidation, in which the Ti element was rejected in the reaction interface. When the content produced during the internal oxidation of the Ti element reached a critical value, dispersed TiO₂ was formed and kept growing to form the continual layer. The alternating formation of continual Al₂O₃ and TiO₂ layers resulted in the layer structure observed in the surface.