An LC-MS profiling method was used for a comprehensive study of the phenolic components of collard greens, kale, and Chinese broccoli, three Brassica green leafy vegetables. This study led to the identification of 45 flavonoids and 13 hydroxycinnamic acid derivatives in the three vegetables. Most of the identifications were based on comparison of compounds previously reported in the literature for Brassica vegetables. The results indicate that the three materials have very similar phenolic component profiles. For each, kaempferol glycosides and acylgentiobiosides were the major phenolic compounds, quercetin glycosides were minor compounds, and most of the flavononol glycosides existed in their acylated forms. In addition, each of the materials contained caffeoyl-, p-coumaroyl-, and feruloylquinic acid monomers with a 3-position derivative as the dominant isomer. This is the first report for most of these phenolics in collard greens and Chinese broccoli and for >20 of them in kale.