DL-alpha-Lipoic acid at low concentrations increases the stability of thiamine hydrochloride solutions, however, at higher concentrations the degradation rate of the vitamin is enhanced. The optimum concentration of lipoic acid at which its stabilizing efficiency is maximum is shifted on the addition of 20 ppm of copper, from 0.5 to 1.0 mmole/1, indicating that this optimum concentration is dependent on heavy metal ions concentration in the solution. In the presence of copper, the degradation rate of the vitamin in solutions containing lipoic acid at concentrations higher than its optimum concentration is lower than that in corresponding solutions in the absence of added copper. On the other hand, below the optimum concentration, the degradation rate of thiamine hydrochloride solutions in the absence of copper is lower than that of the corresponding solutions to which 20 ppm copper have been added. This indicates that the presence of either free copper ions or an excess of lipoic acid is deleterious to the stability of the vitamin.