A process for treatment of acidic waters via membrane separation equipment. A feedwater is maintained or adjusted to a pH of 4.3 or lower, and fed to a membrane separation system. In this manner, species such as total organic carbon (TOC) become more ionized, and (a) their rejection by the membrane separation process is significantly increased, and (b) their solubility in the reject stream from the membrane process is significantly increased. Passage of total organic carbon (TOC) through the membrane is significantly reduced. A recovery ratio at or exceeding eighty percent (80%) is achievable with most feedwaters, while simultaneously achieving a substantial reduction in cleaning frequency of the membrane separation equipment. The method is particularly useful for the preparation of high purity water.