Perlites from large-scale deposits in the USSR were investigated by a modified DTG method. The technique permits an essentially higher DTG curve resolution in comparison with that of conventional DTG curves obtained with a derivatograph. The investigated perlites are classified into two groups: dense and porous. Elimination of water from perlites occurs in from 3 to 5 stages. The content of water lost at low temperature is higher in the dense perlites than in the porous ones.