DTA studies were performed to investigate the polymerization of molecular (yellow) arsenic (v-As), in the dark and also under illumination/irradiation. The semiempirical quantum chemical CNDO/BW method for calculating the dimerization reaction pathway of two As 4 molecules was applied, and showed that the stability of y-As decreases, due to the formation of stable fragments of the polymeric phase (a-As). Using the principle of orbital symmetry conservation, it was shown that polymerization takes place most favourably from a staggered face-to-face configuration of As 4 molecules. Dark polymerization proceeds under activation. For a result of the initial stage of polymerization, CNDO/BW simulation predicts the formation of an As 8 dimer. The molecular cluster As 8 of D 2 h symmetry forms a six-membered chair-shaped ring which constitutes one of the dominating units in the polymeric phase (a-As).