Radiolysis of 0.05% aqueous solution of benzyl alcohol with 50Co gamma-rays ranging from 1 X 10(4) to 7 X 10(5) rad was investigated, in order to presume the change of it contained in radiopharmaceuticals. For both O2 free and oxygenated solutions, an approximately linear relationship holds between the retaining benzyl alcohol and dose in the range from 1 X 10(5) to 7 X 10(5) rads. The G(-M) values of benzyl alcohol calculated from the relation were 2.34 in the absence and 1.92 in presence of oxygen. In the presence of oxygen, a main product was benzaldehyde and its G value was 0.87. In the absence of oxygen, the main products of the radiolysis were dibenzyl, benzyl phenylcalbinol and hydrobenzoin, which were regarded as the radicao-reaction products of PhCH2 and PhCHOH, and the yield of benzaldehyde was negligible. Irrespective of the presence of oxygen, o- and p-hhdroxylated products of benzyl alcohol were found only in small quantity.