The reaction between thin Fe films and gallium arsenide is studied using chiefly cross-sectional transmission electron microscopy. Fe films, 50-120 nm in thickness, were deposited onto (100) GaAs substrates by ion-beam sputtering and the Fe/GaAs couples were then annealed under vacuum at temperatures ranging between 400 and 550 degrees C. The presence of a thin amorphous intermixed layer at the Fe/GaAs interface is pointed out in the as-deposited conditions; this layer consists of three elements Fe, Ga and As. The determination of the residual internal stress in the as-deposited Fe films is also performed using the sin(2) psi method and the result is of the order of -2 GPa. Iron starts to react with GaAs at approximate to 400 degrees C, producing a layered Fe/Fe3Ga/Fe2As + FeAs/GaAs structure. The sequence suggests that Fe diffuses into the GaAs and liberates Ga while forming iron arsenides. After annealing, the Fe/GaAs bimaterial also exhibits interfacial undulations which are discussed in terms of strain-energy relaxation.