The aim of the present work is to improve the sensitivity in the RPLC determination of acyclovir [9-(2-hydroxy ethoxymethyl) guanine] (ACV) and guanine, the major impurity of the drug synthesis and one of the compounds found in the chemical degradation process of ACV. The method was applied to the quantification of drug in liposomal formulations. The most important problem for RPLC analysis of both compounds are their high pKa values, mainly guanine, and the interaction with reactive silanol groups in the stationary phase. In order to avoid these problems there are four basic strategies: (i) ionic pair reagents, (ii) deactivated silica columns, (iii) polymeric based columns and (iv) silanol masking agents. A validation protocol was followed to develop the analytical method, using a Spherisorb ODS (250 x 4.6 mm i.d.) analytical column, with a mobile phase of 95% aqueous phosphate buffer (pH 3.0) and 5% HPLC methanol pumped isocratically at 1.3 ml/min(-1), with ultraviolet detection at 254 nm. The results showed a high reproducibility in retention time value, with R.S.D. of 2.37% for ACV and 0.32% for guanine. The lowest concentration levels assayed, 0.15 microg/ml(-1) for guanine and 1 microg/ml(-1) for ACV, showed good R.S.D. in the quantification parameter (peak area) 11.0% (guanine) and 9.64% (ACV)