The diet of Rhinopithecus roxellana is characterized by lichens, which are available year-round and an uncommon food source for nonhuman primates, supplemented by seasonal plant foods. We present the first study of foods eaten by R. roxellana in relation to nutritional chemistry in Shennongjia National Nature Reserve, Hubei Province, China. We analyzed the nutrients (crude protein, crude fat, and water soluble carbohydrate [WSC]) and feeding deterrents (crude fiber, condensed tannin [CT], and total phenolic [TP]) of 111 parts from 53 plant species and of 6 lichen species. Results showed that lichens were a good choice for R. roxellana living in habitats with limited and seasonally available plant foods. They contained higher concentrations of WSC than foliage, fat concentrations equivalent to those in plant parts (except fruits/seeds), and lower concentrations of fiber than mature leaves, flowers, and fruits. Although lichens were lower in protein than plant parts (except fruits), the monkeys could likely meet their protein requirement by eating seasonal plant foods rich in protein, including foliage, flowers, buds, and seeds. The monkeys were not observed to select foliage higher in protein, but appeared to select mature leaves higher in WSC and lower in fiber. Fruits were a good source of WSC and fat, and seeds were a good source of fat. Neither CT nor TP content showed negative effects on the selection of mature leaves or lichens. Am. J. Primatol. 75:860–871, 2013. © 2013 Wiley Periodicals, Inc.