Abstract A flow-injection analytical method for the determination of hydrochlorothiazide is presented. The method is based on the chemiluminescence reaction of hydrochlorothiazide with cerium(IV) in sulphuric acid, sensitized by the fluorescent dye rhodamine 6G. The proposed procedure allows quantitation of hydrochlorothiazide in the concentration range of 0.33–130 μmol l −1 with a detection limit of 0.15 μmol l −1, an RSD of 2.4% at 10 μmol l −1 and a sample measurement frequency of 200 h −1. The method was successfully applied to the determination of hydrochlorothiazide in pharmaceutical preparations containing, amongst others, lactose, maize starch, calcium phosphate, magnesium stearate, potassium chloride and E 110 (disodium-6-hydroxy-5-(4-sulphonatophenylazo) naphthalene-2-sulphonate) as the concomitant species. Apart from the single formulation, hydrochlorothiazide was also determined in tablets combined with the antihypertensive lisinopril.