The assay of active pharmaceutical ingredients (API) of compounding preparation is needed to assure the effectivity and safety of the pharmaceutical therapy. This research aims to assay determination of API in compounding capsules of aminophylline, chlorpheniramine maleate (CTM), and prednisone. The assay is done by UV spectrophotometry with Partial Least Square (PLS) multivariate calibration to thirty compounding capsules taken from Apotek “X” in Sleman. The result of the assay of aminophylline is 56,983% - 137,89%, CTM - 159,59% - 131,01%, and prednisone 32,31% - 245,79%. The Farmakope Indonesia 6th Edition assay requirement of aminophylline is 93,0% – 107,0%; CTM 90,0% - 110,0%; and prednison 90,0% - 110,0% of the label amount. The result shows that thirty samples of compounding capsules don’t meet the requirements stated by Farmakope Indonesia 6th Edition.