The a -> b phase transformation upon heating in the Ti-5553 alloy with lamellar-nodular bimodal microstructure was tracked in situ with high energy X-ray diffraction. Rapid heating at 10, 50 and 100 °C s-1 from room temperature to 1050 °C was tested. Phase transformation on heating was studied by a combined analysis of the microstructural features that provides estimates of mass fractions, mean lattice parameters and full width at half maximum for the two phases. In comparison with equilibrium conditions, the experimental mass fractions reveal a shift of the transformation domain toward high temperatures when the heating rate increases. Also, the dissolution of the a phase is largely impacted by its morphology, the transformation being faster for a lamellae. The combined analysis of mean lattice parameters and full width at half maximum suggests that the a -> b phase transformation on heating is diffusion controlled. The b phase therefore inherits the solute content of the adjacent parent a phase, leading to chemical heterogeneities in the b phase regardless of the heating rate.