Two new simple and selective assay methods have been presented for the binary mixtures of moexipril hydrochloride (MOEX) and hydrochlorothiazide (HCTZ) in pharmaceutical formulations. The first method depends on second-derivative ultraviolet spectrophotometry with zero-crossing measurements at 215 and 234 nm for MOEX and HCTZ, respectively. The assay was linear over the concentration ranges 1.0-11.0 mug ml(-1) for MOEX and 0.5-9.0 mug ml(-1) for HCTZ. The determination limits for MOEX and HCTZ were found to be 1.0 and 0.5 mug ml(-1), respectively; while the detection limits were 0.2 mug ml(-1) for MOEX and 0.1 mug ml(-1) for HCTZ. The second method was based on isocratic reversed-phase liquid chromatography by using a mobile phase acetonitrile-20 mM phosphate buffer (pH 4.0) (50:50, v/v). Lisinopril was used as an internal standard (IS) and the substances were detected at 212 nm. The linearity range for both drugs was 0.5-12.0 mug ml(-1). The determination and detection limits were found to be 0.100 and 0.010 mug ml(-1) for MOEX and 0.025 and 0.005 mug ml(-1) for HCTZ, respectively. The proposed methods were successfully applied to the determination of these drugs in synthetic mixtures and commercially available tablets with a high percentage recovery, good accuracy and precision. (C) 2003 Elsevier Science B.V. All rights reserved.