The variation of quality and fouling properties of wwtp effluent over time influences the performance of ultrafiltration (UF) plants. This research reports three experiments in which the specific ultrafiltration resistance (SUR) measurement is applied to obtain more insight in quality variation an impact on UF performance. Firstly the overall performance (flux, recovery and trans-membrane pressure) of a UF pilot was compared with the SUR values of the feedwater. Secondly the effect of pre treatment was investigated. At last the relation between the slopes of Filtration cycles of a UF installation and SUR measurements of feedwater were compared. The results clearly indicate that the overall performance of the UF pilot depends on the SUR values of the feedwater. The SUR values should be less than 10x10(12) m(-2) for a stable UF performance. Also the effect of pre treatment depends oil the SUR values of raw wwtp effluent. Very low SUR values (similar to 4x10(12) m(-2)) mean less effect compared to higher SUR values. The relation and correlation between SUR values of feed water and single filtration cycles of UF installations depends on the condition of the membranes in the units. Using fouled membranes the variation of SUR values corresponds to higher TMP variations than using cleaner membranes.