The role of electrostatic interactions in the separation of pharmaceuticals by a loose nanofiltration (NF) membrane was examined. While retention of the non-ionizable pharmaceutical carbamazepine was relatively independent of the solution chemistry, retention of the ionizable pharmaceuticals sulfamethoxazole and ibuprofen was strongly influenced by the solution pH and ionic strength. This finding is consistent with previous results investigating the effects of solution pH and ionic strength on the retention of proteins and organic acids. Pharmaceutical retention increases dramatically as the compound transforms from a neutral to a negatively charged species when the solution pH increases above its pKa value. In contrast, solution ionic strength suppresses the double layer or the Debye screening length and therefore reduces the effectiveness of electrostatic interaction as a major retention mechanism by the loose NF. However, because of the formation of a hydrated layer around the charged functional groups of the pharmaceuticals and the fact that at a sufficiently high ionic strength the Debye length approaches a relatively constant value, this reduction in retention is relatively small. As a result, even at comparatively elevated ionic strengths, retention of the negatively charged sulfamethoxazole and ibuprofen by the loose NF membrane is considerably high.