A fast and reproducible high performance liquid chromatography method has been developed for the determination of diclofenac sodium and its degradation products in commercial and in in-house produced ointments. The method employs a RP-LiChrospher^® select B (C8) column with a mobile phase containing methanol/water (63:37, v/v) and detection at 220 nm. This rapid and simple HPLC assay was used for QA/QC of large scale in-house produced diclofenac gel. The validation protocol was designed following international guidelines, e.g. ICH Q2(R1). Selectivity tests also included the separation of synthesis related by-products like 1-(2,6-dichlorphenyl)indoline-2-one (impurity A) and indoline-2-one (impurity E), and in addition selectivity with regard to several photodegradation products produced by both UV and simulated sunlight irradiation has been shown.