1. The hot-body ignition temperatures under free convection conditions and the dependence of TS on composition were measured for air mxtures of hydrogen, acetylene, carbon disulfide, ethylene glycol ethyl ether, and diethyl ether. 2. It was established that ignition can occur at TS to 180°C which is incompatible with a thermal mechanism. Low-temperature ignition is explained by the formation of cold flames associated with the development of degenerate branching of the chain-reaction process. A reaction of sufficient duration for this occurs only under conditions of free convection. 3. In many cases TS is evidently affected by a two-stage mechanism; in this case the minimum TS may correspond to compositions close to the critical compositons for flame propagation. 4. The results of the investigation confirm the need for a reexamination of existing safety regulations. Permissible temperatures should be determined under hot-body ignition conditions corresponding to the actual use of equipment when cold-flame ignition is a possibilitv. 5. The following permissible temperatures, which incorporate the necessary safety factor, may be recommended: for CS2 and (C2H5)2O−150°, for C2H2−300°, for H2 and ethylene glycol ethyl ether −500°, for B-70 gasoline−700°, and for CH4−800°C.