Dioctahedral vermiculite commonly occurs in soils and fresh sediments, but has not been reported in sedimentary rocks. Little is known of the evolution of this mineral during diagenesis. According to the available literature, dioctahedral vermiculite is likely to exhibit strong potential for selective sorption and fixation of K+ involving interlayer dehydration and collapse. The objective of the present study was to investigate the influence of K+ saturation and seawater treatments on the structure of dioctahedral vermiculite. Due to the fact that no dioctahedral vermiculite standard reference material was available, a natural sample of soil clay containing dioctahedral vermiculite was used in the study. The clay was saturated with K+ using different protocols simulating natural processes taking place in soils and marine environments. The solid products of the experiments were analyzed for potassium content using flame photometry. The effect of the treatments used on the structure of dioctahedral vermiculite was studied using X-ray diffraction (XRD). The percentages of the collapsed interlayers were estimated by modeling the XRD patterns based on a whole-pattern multi-specimen modeling technique. All the treatments involving K+ saturation caused K+ fixation and irreversible collapse (i.e. contraction to 10 Å ) of at least a portion of the hydrated (vermiculitic) interlayers. Air drying of the K+-saturated samples greatly enhanced the degree of the collapse. The results obtained gave no clear answer as to whether time had had a significant effect on the degree to which irreversible collapse occurred. Selective sorption of K+ from artificial seawater was observed. These results clearly indicate that collapse of dioctahedral vermiculite is likely to occur in soils during weathering and in sediments during early diagenesis. Both processes need to be taken into consideration in sedimentary basin studies.