High-temperature in situ 57Fe Mössbauer spectroscopy was used to investigate the nanocrystallization process in Fe73.5Si 13.5B9Cu1Nb1X2 (X=Zr, Nb, Mo and V) amorphous alloys. The Mössbauer spectra were taken at temperatures where only the amorphous phase is found in a paramagnetic state, allowing an accurate resolution between the amorphous and crystalline phases presented in the annealed ribbons. The Johnson-Mehl-Avrami theory was used to describe the increase of the crystalline fraction (in vol.%) as a function of the annealing time. The volume fraction of the iron-silicate nanocrystalline phase, found in each of the alloys annealed under similar conditions, increases in the sequence Zr<Nb<Mo<V. The value of the Avrami exponent, determined for each alloy, is near 1, indicating a controlled diffusion process with a nucleation rate close to zero. An activation energy of 3eV was found for the studied alloys.