This paper develops a typology for purchasing groups. In the typology, five main forms of cooperative purchasing are distinguished based on seven main dimensions. The forms are positioned in a matrix according to two distinguishing dimensions. These two dimensions are the ‘influence by all members on the group activities’ and the ‘number of different group activities’. Underlying the two-dimensional matrix, there are five other dimensions that do not distinguish all forms from each other, but further detail the forms of cooperative purchasing. The typology can serve as a guideline for purchasing groups when a suitable organisational form needs to be chosen. In a suitable form, the dimensions of a group fit together. For all groups, it is recommended to find this best fit. This is something in which the typology may help. The paper concludes by emphasising the importance of clearly defining and positioning studied forms, because different forms imply different research models and have different advantages, disadvantages, and critical success factors.