The geochemical transformations of a number of individual model sulfur compounds were investigated using laboratory hydrous pyrolysis under conditions of constant temperature between 200 and 330°C and in the presence of low-sulfur brown coal. The results show that most of the compounds were reactive and could eventually be transformed largely into secondary sulfur species, whereas only aryl sulphides and condensed sulfur compounds were virtually inactive. Analysis of sulfur species in the pyrolysis products of the reactive compounds suggests that the geochemical transformation of organic sulfur could be principally characterised in terms of the formation and distribution of hydrogen sulphide, secondary sulfur compounds and macromolecular sulfur. The great variation in the proportions of secondary sulfur species indicates that there are significant differences in the geochemical behaviour of the various sulfur compounds or structures. However, as hydrogen sulphide is the predominant pyrolysis product, its formation from the decomposition of organic sulfur compounds proves to be the primary and the most significant mechanism for organic sulfur transformation during the maturation process.