The magnetic and structural characteristics of FeSiBCuNb alloys have been studied for samples submitted to thermal treatments at increasing temperatures up to 650 o C. The study has been performed on alloy wires obtained by the in-rotating-water quenching-technique that exhibit an amorphous structure in their as-cast state. Particular attention is paid to the influence of separate additions of Cu and Nb to the reference FeSiB alloy, although a detailed analysis on the role of each element is disclosed. It includes overall information on the grain size and nature of crystallites for the different devitrification stages and the correlation between magnetic and structural characteristics also after annealing at low temperatures where first crystallization stages are detected.