Nanocrystalline phases can be formed during crystallization of amorphous alloys annealed isothermally below the crystallization temperature. We present magnetic susceptibility data on this transformation. Magnetic measurements are able to detect the onset of the transformation of amorphous Ni-P alloys much earlier than differential scanning calorimetry. The transformation kinetics are analyzed by means of the Avrami plot based on the Johnson-Mehl-Avrami equation. The kinetics of further solid state reactions in the nanostructured material can be investigated similarly.