The formation and development of Mo-Si interfaces in MoSi multilayers upon thermal annealing, including a transition to h-Mo Si2, have been investigated using high resolution transmission electron microscopy, x-ray reflectivity, and x-ray diffraction measurements. The silicide layers naturally formed at Mo-Si interfaces, i.e., just upon and after the deposition, are amorphous and have different thicknesses for as-deposited samples, with the Mo-on-Si interlayer being the largest. In addition, silicide growth at Mo-Si interfaces during annealing before the phase transformation predominantly takes place at the Mo-on-Si interface and a Mo Si2 interface layer is formed. Diffusion continues until a thick Mo Si2 layer is formed at the interface, at which point the interface crystallizes and diffusion speeds up, finally resulting in an abrupt intermixing and phase transition of the entire interface to h-Mo Si2. This model predicts an onset of the phase transition which does not depend primarily on the annealing temperature but on a threshold thickness of the Mo Si2 interface before crystallization takes place. This crystallization threshold is shown to exist not only in the MoSi system, but also occurs for MoSi multilayers where the natural interfaces are replaced by diffusion barriers.