The solubility of gypsum in organic acids namely acetic, oxalic, tartaric and succinic acids at low temperature (30 degrees C) was studied. The results show that sulphate sulphur content increases with increasing acid concentration from 0.1 to 0.25 M and decreases again at higher concentration. It is suggested that different behaviour of the acids beyond 0.25 M solution could be due to incorporation of sulphate into the co-ordination sphere of calcium-organic complexes. It is suggested that the occurrence of free and fixed sulphate in the solution is highly pH dependent. The nature of incorporation of sulphate in a system containing calcium species and natural organic matter has also been studied. Humic acids, extracted from forest and tea-garden soils, were treated with a solution of gypsum (CaSO4. 2H(2)O) at room temperature and the infrared spectra of the gypsum treated samples reveal that sulphate has been incorporated into the calcium complex as a monodentate ligand. Further an inorganic-sulphur free high-sulphur coal when treated with the gypsum solution, incorporated sulphate as bidentate ligand. It is concluded that the nature of the organic matter plays an important role for the occurrence of various fixed sulphates either as monodentate or bidentate ligand in natural systems. This work is a novel breakthrough for the occurrence of varying sulphate content in some of the natural environments and has considerable environmental and geochemical interest.