Recirculation systems are becoming widely used in the aquaculture industry, where ammonium removal results are crucial for a proper performance considering its high toxicity on fish. The objective of this study was to assess the ammonium retention, when the transmembrane pressure (TMP) in nanofiltration (NF) and reverse osmosis (RO) membranes are changed. Two pH levels were tested: pH 5, where ammonium is as NH4+, and pH 7 which is an average of the most common range in aquaculture (pH 6-8). In this study a model solution of ammonium-water and commercial membranes of polyamide supplied by Alfa Laval (DSSHR98PP) and Dow (NF90, NF200 and NF270) were used, with a membrane area of 0.01018m2. A critical TMP was determined, where the repulsive electrochemical forces at the active layer were exceeded by the transmembrane pressure, causing an irreversible decrease of the retention values. The maximum TMP values for NF and RO membranes were 16bar and 24.5bar, respectively. Ammonium retention at pH 7 was higher than pH 5 for all assessed membranes before the critical TMP was reached, with average values of 100, 99.7, 98.4 and 87.5% for membranes NF90, DSSHR98PP, NF200 and NF270, respectively. It may be concluded the ammonium retention mechanism in the studied membranes basically corresponds to an electrochemical mechanism, where pH and critical TMP influence the retention values of ammonium.