It has been known for a long time that combustible bulk materials may undergo self-ignition if stored in sufficient amounts at adequate ambient temperatures. Most of the studies according to this problem refer to atmospheric ambient conditions (oxygen volume fraction of 21%). In some technical applications, however, bulk materials are processed or stored at a reduced level of oxygen concentration. Besides the oxygen volume fraction, the moisture content of the bulk material itself as well as the humidity of the ambient air effect self-ignition. The first part of the current paper presents experimental investigations on the influence of the volume fraction of oxygen on the self-ignition temperature of the bulk materials. The lower the oxygen volume fraction in the surrounding gas, the higher is the self-ignition temperature as a general trend. In the second part, tests under a normal atmosphere were carried out to investigate the influence of water in a liquid and vapour state on the self-ignition procedure of lignite coal. They showed that sub-critical deposits could become super-critical by pouring water into the bulk or by exposing a dry bulk sample to humid air. Besides the experiments, a numerical model was established which allows to compute the process of heating and self-ignition in bulk deposits. Technical applications of the model cover safe storage of dusts, granulate, bulk materials and wastes.