Microorganisms can reproduce only in such substrata which contain a certain amount of water and dissolved nutritious substances. Therefore, it is necessary to search for such charges that are distinguished by a large filtering surface and good humidity-retaining properties. In experiments three types of materials - wood, zeolite and foam - were used as a charge for the biological air-cleaning process. The charges were mixed up to maintain humidity necessary for the microorganisms in the biofilter as long as possible. The charges were placed into three different cassettes in the biofilter. The wood charge consisted of wood chips of 10-15 mm and that of zeolite contained 10-12 mm of zeolite granules. The foam rubber was cut into separate cubes each having the dimensions of 30×30×20 mm. Dependence of the charge humidity on the airflow rate, temperature, time and charge thickness was determined in the work. During the experiments the airflow rate reached 0.1-0.6 m/sec., and the temperature ranged from 15 to 35 °C. Foam showed the best sorption properties, but a better distribution of humidity was found in the wood-chip charge.