Various metals (gold, silver, brass, copper and bronze) were exposed to humid and dry hydrocyaric acid under controlled conditions. Formation of layers and deposits were studied by scanning electron microscopy (SEM). Layer formation was found to be strong with humid HCN and faint or negligible in dry HCN atmospheres. Some of the reaction products were identified by x-ray diffraction as metal cyanides. To reduce or prevent layer formation on metals during fumigation with HCN, the relative humidity inside rooms or buildings should be reduced prior to fumigation by drying agents or heaters.