Numerous measurements of 131I concentration in milk following the Chernobyl accident have shown a wide range of clarification half-life values. By results of spectrometric measurements of the milk, performed in 1986 in Tula region (Russia), a connection between a 131I decreasing half-life in milk and a level of radioactive contamination in a locality and a level of precipitation during passage of a radioactive cloud is analysed. Values of the 131I half-life in milk increase from 3.0 d at the smallest area contamination by 137Cs to 5.5–6.0 d at the contamination above the 200 kBq/m2. The half-life of 131I removal from milk depends on the precipitation level at the time of radioactive fallouts in places of milk sampling. Tef increases from 2.8–3.5 d (dry deposition) to the value 5.5–6.0 with precipitation increasing up to 6–9 mm. The half-life value does not change with further precipitation increase. This finding should be taken into account in thyroid dose estimations for inhabitants of contaminated areas.