A selective high-performance thin-layer chromatographic (HPTLC) method has been established for the quantitative determination of oseltamivir phosphate (OST) without interference of ascorbic acid (ASC) added to some of its counterfeit pharmaceutical formulations. Chromatographic separation was performed on precoated silica gel 60 GF254 plates with methanol-water-ammonia 6:4:0.05 (7p/7p) as mobile phase at ambient temperature. The developed plates were scanned and quantified at 254 nm. Experimental conditions such as band size, mobile phase volume, chamber saturation time, migration of solvent front, etc. were critically studied, and the optimum conditions were selected. Asatisfactory resolution was obtained with RF 0.70 and 0.83 for OST and ASC, respectively. Also, HPTLC-band detection method has been established for rapid qualitative assay of OST using ninhydrin spray. On the other hand, a colorimetric method has been established using bromocresol green (BCG) as rapid, accurate, and selective comparative...