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A simple HPLC-UV method was developed and validated for the quantitation of RP free base encapsulated into two new multiparticulate systems (microparticles and nanoparticles), as well as for the quantification of RP hydrochloride when given as a loading dose together with the new delivery system developed. HPLC separation was achieved using a C18 Kromasil column (250 mm × 4 mm) with a mobile phase composed of acetonitrile-phosphate buffer solution (55:45, v/v) adjusted at pH 6.0 and containing 0.3% triethanolamine. Flow rate was set at 1.0 mL min−1. The UV detector was operated at 245 nm. The method allowed for the simultaneous determination of both RP and RP-HCl. The method was linear within the range 2.5–50 μg mL−1 for both RP and RP-HCl. The limits of detection (LOD) and quantitation (LOQ) found were 0.8 μg mL−1 and 2.4 μg mL−1 for RP, and 0.3 μg mL−1 and 0.9 μg mL−1 for RP-HCl. The method was found to be simple, rapid, specific, precise, accurate, and reproducible. The method was successfully applied to the determination of the encapsulation efficiency of RP in the multiparticulate systems developed, being 85.03 ± 3.77% and 51.12 ± 3.50%, for RP-loaded PLGA microspheres and RP-loaded PLGA nanoparticles, respectively.