Since early 2000, a type of group-based informal irrigation for irrigating a very small area – less than two hectares on average, has seen widespread adoption among Malawian farmers. The technology, called “temporary” irrigation, uses farmers’ own labor and locally available materials for constructing river diversion structures and canals, and these are managed by informal “clubs.” This paper attempts to assess the effectiveness and sustainability of this technology from the beneficiaries’ point of view, by employing an analytical framework that focuses on property rights and collective action as critical factors affecting technology choice and adoption. A comparative examination of selected informal irrigation cases in the Dowa district reveals that, in spite of the absence of secure tenure over land and water and of strong collective-action incentives among farmers, temporary irrigation has satisfied most of the effectiveness criteria and thus has contributed to technological expansion. This is attributable to the relative resource affluence and temporary nature of irrigation facilities which have existed at least up to the present. But the very “success” of the informal irrigation technology is changing the nature of the resources: where water in the streams has become scarce, river-bank lands have been commoditized and temporary diversion structures have been upgraded to permanent structures. Specific policy measures necessary to ensure sustainability of temporary irrigation should include: 1) provision of agronomic extension services to improve the profitability of irrigation, 2) promotion of basin-wide watershed management including provision of opportunities for stakeholder dialogues for conflict resolution while improving water use efficiency to enable secure and equitable access to productive resources, and 3) a cautious approach in strengthening water users' associations based on deeper understandings of the farmers’ incentives (cost and benefit) for taking collective action.