Spatially resolved X-ray diffraction (SRXRD) technique has been applied using synchrotron radiation to map the phases present in fusion welds in situ with a sub-millimetre resolution. For titanium, exhibiting an allotropic transformation from a hcp alpha-phase to a bcc beta-phase at similar to 922 degrees C, the following results have been obtained for the heat-affected zone (HAZ). (i) Go-existence of both alpha-and beta-Ti phases in the HAZ as derived from the phase concentration profiles. (ii) The width of the HAZ was found to be 3.33 +/- 0.33 mm as defined by the existence of the high-temperature bcc phase in this zone. (iii) Peak profile and ex-situ post-weld ESCA analyses revealed additional hcp patterns attributable to two TiOx phases formed in an overlayer during the welding process by oxygen diffusion from the ambient at high temperature. The thickness of this TiOx overlayer varied from 50-85 nm, increasing towards the weld pool. The SRXRD technique provides real-time chemical dynamics and in situ phase mapping data for modelling of kinetics of phase transformation and microstructural evolution in allotropic and other more complex systems, under steep thermal gradients and non-isothermal materials processing conditions.