[en] A time-sharing dual wavelength spectrophotometer with phasesensitive demodulation is briefly explained in respect to its main components and function. Using yeast hemoglobin from Candida species as an oxygen indicator, this spectrophotometer reaches a high resolving power in measuring low oxygen concentration in an unstirred gas-liquid system in steady state established by diffusion. Measurements of the dissolved oxygen in tissue culture medium which was layered over cells and exposed to X-radiation with a low dose rate, failed to indicate any radiochemical consumption of oxygen