The reaction of Cr/Se multilayers when they are annealed occurs in two steps: interdiffusion of the single layers to an amorphous Cr-Se alloy and crystallization of Cr3Se4. Both reaction steps were characterized using various techniques. At approximately 300 degrees C the layers have interdiffused completely to form a homogeneous amorphous Cr-Se alloy. Short-range order in the alloy was probed with X-ray absorption spectroscopy (XAS) and, according to the results of this, is already very similar to Cr3Se4, which crystallizes around 500 degrees C. Crystallization occurs at a well-defined temperature, whereas crystallite growth proceeds in the whole temperature interval above the crystallization temperature and is not finished at 660 degrees C. The reaction yields a polycrystalline thin film of Cr3Se4 in a preferred orientation exhibiting a (00 texture. In Cr-rich samples amorphous Cr is present as a by-product. A Cr-Se/Se multilayer was observed as an intermediate in the interdiffusion of some Cr-rich samples which is stable between 200 and 250 degrees C.