During the last 10 years the ignition behavior of combustible clouds of sprays, powders and fibers (so-called hock) has been intensively investigated at PTB within the scope of several experimental research projects. The investigation covered sprays of organic solvents, oils and multiphase systems (e.g. water-based paints) as well as clouds of coating powders and flock. The liquid spray clouds were mainly produced by electrostatic high-rotation bell-type sprayers, the solid I,article clouds were obtained by dispersing particles with air pressure. The resulting clouds were ignited by electric sparks (with and without additional inductance), arcs (AC and DC), chemical igniters and open flames at their optimum ignition concentrations as well as at their flammability limits. The experimental results obtained enhance the knowledge about the ignition behavior of combustible clouds by ignition sources. Pictures, tables and equations are presented which describe the phenomena found and allow their application in similar situations. (C) 2001 Elsevier Science Ltd. All rights reserved.