Lignin hydrolysates contain many different chemical species, including ferulic acid, coumaric acid, vanillic acid, vanillin, syringaldehyde and furfural. From the perspective of biofuels, these compounds are problematic and can cause downstream loss of product if not removed prior to beginning the fermentative process. In contrast, a search for these compounds within the literature turns up many papers where the same compounds have beneficial properties pertaining to human health, including as antioxidants and in cancer prevention, or are involved in bacterial cell-to-cell signaling. Consequently, this article reviews the dual nature of these and other compounds found in lignin hydrolysates, highlighting both their detrimental and beneficial activities.