Abstract: The invention concerns a monitoring unit [10] for monitoring the condition of a semi-permeable membrane [24] in a water purification system, and particularly the condition of spiral membrane in a reverse osmosis water purification system. The monitoring unit [10] comprises a flow chamber [12] that includes an inlet for permitting ingress of a feed fluid into the flow chamber [12], and a feed fluid outlet [22] for permitting at least partial through-flow of the feed fluid through the flow chamber [12] such that cross-flow conditions apply in the flow chamber [12]. The semi-permeable membrane [24] is at least partly supported in the flow chamber [12].