The β2-α1 "Bainitic" transformation in a quenched Ag-45 at.% Cd alloy is representative of a group of solid state phase transformation in which the metastable product phase exhibits the geometry and crystallography of a displacive, diffusionless transformation, although formation of the equilibrium α phase requires a composition change involving long range diffusion. In the present work, energy dispersive X-ray microanalysis has been used in an attempt to determine the point at which a change in composition is detectable within the product phase during isothermal ageing. When care is taken to minimize the influence of thin foil surface films, the results indicate a significant difference in composition between the α1 plates and β2 matrix at the earliest stages of transformation examined (30 s ageing at 240°C). The heavily faulted, freshly formed plates have a disordered, 9R crystal structure and a composition equivalent, within the limits of experimental error, to that of the equilibrium α phase. It is implied that the α1 plates form with near to the equilibrium composition or achieve it immediately after initial plate formation and before any significant recovery of the plate substructure or the 9R to 3R transition in crystal structure that accompanies extended ageing.