The immobilization of high level liquid wastes from the repro-cessing of irradiated nuclear fuels is of great interest, and serious efforts are being undertaken to find a satisfactory technical solution. Volatilization of fission product elements during immobilization poses the potential for the release of radioactive substances to the environment and necessitates effective off-gas cleaning systems. This report describes typical off-gas cleaning systems used in the most advanced high level liquid waste immobilization plants and considers most of the aerosols and volatile contaminants. Contents: 1. Introduction; 2. Nature of high level liquid waste (HLLW); 3. HLLW solidification processes and their application; 4. Sources and characteristics of off-gas contaminants; 5. Equipment used in off-gas treatment systems; 6. Typical off-gas systems and their performance; 7. Safety considerations; 8. Conclusions.