The temperature dependence of the magnetic susceptibility of a condensed selenium phase is obtained. The variation of the magnetic susceptibility of selenium at 800-1200 degree K is attributed to the paramagnetism of the polymer chain cutoffs. The temperature dependence of the average length (l) of the selenium polymer chain is determined in this temperature range (at 1200 degree K l equals 64 atoms). The fission enthalpy ( DELTA H) of the selenium chain is found to be equal to 33 kcal/mol. The discrepancy between the Se-Se bond energy and DELTA H is attributed to the formation of ″three-electron bonds″ at the ends of the selenium chains.