The separation of formic (C-1) acetic (C-2), propionic (C-3), and n-valeric (C-5) acids in binary water solutions has been studied using a reverse osmosis (RO) membrane pilot plant operating at different temperatures and pressures (usually 2 1 degreesC and 1.72 MPa). The RO membrane, which is composite, polyamide and spiral-wound, having a surface area of 2.6 m(2), was fed by a solution of 0.5 M of C-1 C-2 and C-3 acids and 0.146 M of C-5 The pilot plant was used to simulate a cascade series of RO modules by recycling the permeate flux at the end of each semi-batch run. The influence on the retention coefficient, R, of molecular weight and molar volume, pK(a), of the different acids was determined. For acetic acid the influence of temperature (7) and transmembrane pressure (DeltaP) was also studied, obtaining an inverse and direct good linear correlation for R vs. T and R vs. DeltaP, respectively. The results are particularly interesting for acetic acid-water separation, which is an open question in industrial applications.