For the development of hydrogen phosphide, which has proven to be the most successful gas for tobacco storage sanitation, several products have been designed which avoid completely any contact of the fumigant with the stored product and even allow fumigation at weather conditions where this was not possible earlier. However, to gain full efficacy in killing the insects, certain considerations must be given to the (c.t.)-product, e.g. the proper combination of fumigant concentration and fumigation time. Some negligence in this very point in developing contries could have been the reason of today's not fully proven findings about increasing tolerance of some species towards hydrogen phosphide. Some background information about the currently accepted mechanism of insect intoxication by hydrogen phosphide will be given together with mathematical explanations of gas evolution and gas losses by diffusion.