The thermodynamic and structural aspects of amorphous titanium silicide formation by solid-state reaction have been addressed. The radial distribution function of an amorphous Ti60Si40 reaction product has been determined by electron diffraction. It is an amorphous metallic alloy, with a coordination number of about 11.4, and a local arrangement of atoms which is similar to that in a crystalline silicide, Ti5Si4. The free energy of the amorphous alloy as a function of composition has been determined from the heats of crystallization of co-deposited amorphous Ti-Si alloys, and the heat of formation of the amorphization product as measured by differential scanning calorimetry (DSC). The latter was measured to be 35 ± 5 kJ/mole. Using these data, free energy of the amorphous Ti-Si phase has been calculated by applying the CALPHAD method. These results are discussed in light of the mechanism and energetics of the amorphization reaction.