The nature of structural changes caused by laser writing on metal-silicon thin-film bilayers has been investigated by transmission electron microscopy and electron diffraction. Bilayers of amorphous Si and metals such as Au, Co, Pd, Pt, V, and Rh were reacted by laser writing for periods as long as 0.1 sec and as short as 5 nsec with varying powers by use of neutral density filters. Silicides formed by short laser pulses in bilayers on solid substrates are generally amorphous, while silicides formed on thin membrane substrates require less power for threshold writing and can be crystalline due to lower quench rates. Metallurgical reactions such as grain growth, interdiffusion, and compound formation, which normally require thermal anneals of up to 900°C, have been observed to take place under laser writing with pulses as short as 5 nsec. Writing with excess power results in melting and hole formation as well as a sharp decrease in reflectivity. The maximum change of reflectivity can be obtained at proper (amorphous or crystalline) silicide formation. Effects of film composition and layer thickness on writing power and reflectivity change were investigated in Au-Si and Pd-Si. From all the systems investigated Rh-Si appears to be most promising due to its relatively low writing power and excellent archival properties.