Six environmental standards, Heptachlor, Heptachlor epoxide, beta-Endosulphan, Dieldrin, Endrin and Endrin ketone, have been investigated by DSC. The temperature, enthalpy and entropy of the phase transitions were determined. The plastic-liquid and normal crystal-plastic phase equilibria were used for the determination of eutectic purity by the cryometric method. The plastic phase-liquid equilibrium is not useful for the purity determination. The eutectic purity calculated on the basis of the normal crystal-plastic phase transition is consistent with GC and HPLC results.