RADIATION ‘dose’ is usually defined and measured in terms of the ionization produced in air. However, for biological dosage purposes it would be theoretically preferable to measure effects produced in media similar in composition to living tissue, and there are considerable experimental advantages in a direct measurement of the radiation energy dissipation in liquid or solid media1,2. Our experiments, with X-rays, gamma-rays and beta-rays, confirm that radiation absorption in aqueous solutions can be accurately measured by purely chemical means.