The formation of silicide islands during solid-state reactions of thin films of Pd-W, Pd-Ta and Ni-Nb metallic glasses with Si substrates was studied by transmission electron microscopy (TEM). Pd2Si or NiSi2 islands were found to grow epitaxially in Si as a result of Pd of Ni diffusion from amorphous metallic alloys. The crystallization of films results in the rapid increase of their diffusion inside the substrate, leading to an increase of Pd2Si islands thicknesses which previously grew parallel to the film-Si interface. Silicide islands are mainly bounded inside the substrate by Si(111) planes. Some islands ("negative") were observed to dissolve with time showing the change of the reaction direction. This fact is explained by the possible change of the sign of the Gibbs energy variation Delta T during interface reactions of amorphous metallic films with Si. Some "negative" Pd2Si islands were observed to move during reactions forming channels parallel to film-Si interfaces.