A determination of the number of &agr;‐particles emitted per unit areas per unit time by materials usually used in the construction of ionization chambers has been made. The radioactive contamination of a few gases was examined and methods of removing radioactive material from surfaces were investigated. A FP54 vacuum tube, operated at about one‐tenth of the maximum sensitivity, was used with a photographic recorder for detecting the &agr;‐particles. With this sensitivity &agr;‐particles of a range in air of 2 mm or more could be detected. Cold rolled steel which emitted about 3 &agr;‐particles per 100 cm2per hour was the freest from contamination of any material examined. Brass and copper were the next best materials. Careful sandpapering of a surface was very effective in removing radioactive material. It was also found that a clean surface when exposed to air became contaminated. This made it necessary to keep such surfaces in an atmosphere of nitrogen or some gas which was free from radioactive contamination. Celluloid which was cleaned with alcohol and painted with good drawing ink made a satisfactory material for ionization chamber windows. Amber emitted no more &agr;‐particles than steel but Pyrex glass emitted about 5 times as many &agr;‐particles as steel. Methyl bromide, good quality carbon dioxide, and nitrogen were found to be suitable gases for filling ionization chambers.