Four different cultivars of Chinese Brassica vegetables (two pak choi cultivars and two Chinese leaf mustard cultivars) were fermented according to a traditional Chinese method called pickling. The plant material was investigated before and after the fermentation procedure to determine the qualitative and quantitative changes in its polyphenols. A detailed description of the identified phenolic compounds of leaf mustard by HPLC-ESI-MS(n) is presented here for the first time, including hydroxycinnamic acid mono- and diglycosides (gentiobioses) and flavonoid tetraglycosides. Flavonoid derivatives with a lower molecular mass (di- and triglycosides) and aglycones of flavonoids and hydroxycinnamic acids were detected in fermented cabbages compared to the main compounds detected in nonfermented cabbages (tri- and tetraglycosides of flavonoids and hydroxycinnamic acid derivatives of malic acid, glycoside, and quinic acid). During the fermentation process, contents of flavonoid derivatives and some hydroxycinnamic acid derivatives were found to decrease. Some marginal losses of polyphenols were observed even in the kneading step of the plant material prior to the fermentation procedure. The antioxidative potential of fermented cabbages was much higher compared to that of nonfermented cabbages in the TEAC assay, but not observable in the DPPH assay. The increase of the antioxidative potential detected in the TEAC assay was attributed to the qualitative changes of polyphenols as well as other reductones potentially present.