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The nonsteroidal anti-inflammatory drug diclofenac is widely used. Diclofenac is extensively metabolized to several hydroxylated derivatives and their conjugates. The lactam-dehydrate of 4′-hydroxy diclofenac (4′-OHDD) has now been detected as a new urinary metabolite of diclofenac. Isolation was successfully performed using preparative HPLC in three different steps using water, methanol, and acetonitrile, respectively. The structural characterization of 4′-OHDD was achieved by LC–NMR–MS. In addition, specific mass fragmentation pattern could be obtained using LC–high-resolution MS with both positive and negative electrospray ionization.