This article analyzes the efficiency of Water Users Associations (WUA) in the Cap Bon region (Tunisia) and studies its main determinants. The analysis is performed in two stages. First, the efficiency is measured via the nonparametric “Data Envelopment Analysis” (DEA) technique. The DEA models are constructed not only to assess the overall WUA efficiency but also to evaluate the management and engineering sub-vectors efficiencies separately through a mathematical modification in the initial DEA model. In a second stage critical determinants of efficiency are determined using a Tobit model. In this analysis the focus is on technical (characteristics of the irrigation area and network), organizational and administrative variables. Results show that on average 18.7% of the used inputs could be saved if the WUA would operate on the frontier. The average scale efficiency, which can be calculated as the ratio between Constant and Variable Returns to Scale efficiency measures was around 71%, indicating that many WUAs are not operating at an efficient scale. Subvectors efficiencies show that WUAs present better performances in maintenance activities than in management. The inefficiency found can furthermore be mainly attributed to the number of years of experience in operating a WUA in addition to the number of water pumping stations managed and the rate of the exploited area. The scale inefficiencies are mainly due to administrative and organizational variables.