The degradative reaction of phenobarbital was investigated over a pH range of 6.0 to 10.5 from the standpoint of chemical kinetics and the following results were obtained. 1) The degradation is an apparent unimolecular reaction at any fixed pH value and is in itself a bimoleculer reaction catalyzed by hydroxyl ion. The presence of any catalysis besides that by hydroxyl ion is not recognized. The relation between log k and pH is given in Fig. 4. 2) The results obtained well agreed with Eq. (8) which is derived from the postulation described in the theoretical consideration. As seen in Table III, the velocity constant k₁ (molecular form) is larger than k₂ (ionized form), but the decomposition depends on the concentration of hydroxyl ion, and hydrolysis in an alkaline region is very much accelerated. 3) The velocity constant k₂ was slightly smaller than that calculated and further investigation may be necessary for the elucidation of this reason.