The time-temperature-transformation (TTT) curve for the δ → α′ isothermal martensitic transformation in a Pu-1.9 at. % Ga alloy exhibits an anomalous double-C curve. Recent work suggests that an ambient temperature conditioning treatment enables the lower-C curve. However, the mechanisms responsible for the double-C are still not fully understood. When the δ → α′ transformation is induced by pressure, an intermediate γ′ phase is observed in some alloys. It has been suggested that transformation at upper-C temperatures may proceed via this intermediate phase, while lower-C transformation progresses directly from δ to α′. To investigate the possibility of thermally induced transformation via the intermediate γ′ phase, in situ x-ray diffraction at the Advanced Photon Source was performed. Using transmission x-ray diffraction, the δ → α′ transformation was observed as a function of time and temperature in samples as thin as 30 μm. The intermediate γ′ phase was not observed at -120°C (upper-C curve) or -155 °C (lower-C curve). Results indicate that the bulk of the α′ phase forms relatively rapidly at -120 and -155 °C.