Vaporization of mercury from organomercury cornpounds has been followed by atomic absorption method by the use of long absorption tube shown in Fig. 1. When the solution containing phenyl mercuric acetate was exposed to U.V. radiation, absorbance at 2537 Å of mercury resonance line increased with time, but no increase was observed on the neighbouring line. One of the causes of the loss of organomercury compounds may be explained as the vaporization of mono-atomic mercury produced by U.V. radiationfrom organomercury compounds.