Photo- and thermal-stability of the anthelmintics Albendazole, Mebendazole and Fenbendazole as in solid as in solution form has been investigated, by using a Xenon arc lamp as a radiation source, according to the ICH guideline for the drug stability tests. The degradation process was monitored by a HPLC method. All drugs showed high photosensitivity in solution but a reliable stability in solid form and when exposed to a temperature up to 50 degrees C. Two main degradation products from hydrolysis of the carbamic groups were identified by GC-MS. Validation studies demonstrated high accuracy (recovery 94 to 106%) and precision (RSD under 4.6%) of the HPLC method. The analytical procedure was successfully applied to the control of the drugs in the respective pharmaceutical formulations.