The potential impact of coagulants on mineral scaling in reverse osmosis (RO) feed treatment of brackish water was assessed experimentally, with respect to calcium sulfate and barium sulfate scaling potential, via a bulk crystallization and membrane scaling tests. The scale suppression effectiveness of six commercial antiscalants was first ranked based on measured observed bulk crystallization induction times. Bulk crystallization tests with three standard coagulants (ACH, FeCl3, and polyDADMAC), when used independently, demonstrated retardation of the observed crystallization induction time. However, along with an antiscalant dosing, antiscalant scale suppression was significantly reduced and scaling occurred at the same or greater severity relative to the additive-free feed solution. RO membrane scaling tests also indicated that, when the coagulants were present, antiscalant effectiveness was significantly reduced, consistent with the expectation based on bulk crystallization tests. Although the coagulants demonstrated a slight scale suppression quality in bulk crystallization tests at low dosages (< 10 mg/L), membrane tests, even at a low dose of similar to 3 mg/L, demonstrated scale formation that was at the same or greater level relative to the additive free solution. Optical imaging of the membrane surface demonstrated that, in essentially all cases, greater surface scaling was linked to a higher surface number density of mineral crystals. The percent flux decline increased with increased percent of scaled membrane area. Percent flux decline that was lower than the percent area scaled was encountered for scaling tests without additives and for the case of ACH along with added antiscalant, suggesting partial permeation through the gypsum rosettes. Scaling tests with ACH and with the antiscalant along with FeCl3 or polyDADMAC revealed higher percent flux decline than the percent scaled membrane area. This behavior was linked to scaling by barium sulfate as indicated by SEM imaging and elemental surface analysis and possibly the complexation and/or adsorption of the coagulant onto the membrane surface. The results of the present study suggest that, when feed treatment includes the use of coagulants, excess antiscalant may be needed in order to offset the adverse impact of the coagulants on antiscalant effectiveness.