The high temperature phase transition of muscovite-2M(1) was investigated by "in situ" isothermal Xray powder diffraction data, The results confirm that the reaction taking place in temperature range 700-1000 degrees C is truely a dehydroxylation process, involving nucleation and growth stages of the dehydroxylated phase having Al in 5-fold coordination, The structural simulations agree with the structural model originally proposed by Guggenheim el al. for the dehydroxylated high temperature phase. The kinetic mechanism of the reaction has been interpreted in the frame of the Avrami model, with diffusion as the rate limiting step. The experiments in the upper temperature range (similar to 1000-1200 degrees C) indicate that mullite is the final high temperature product in the reaction pathway, and the process of mullite formation overlaps and interferes with the kinetics of the dehydroxylation process.