Technical Abstract: Distillers' Dried Grains with Solubles (DDGS) and wet distillers’ grains are the major co-products of the dry grind ethanol facilities. As they are mainly used as animal feed, a typical compositional analysis of the DDGS and wet distillers’ grains mainly focuses on defining the feedstock’s nutritional characteristics. With an increasing demand for fuel ethanol, the DDGS and wet distillers’ grains are viewed as a potential bridge feedstock for ethanol production from other cellulosic biomass. The introduction of DDGS or wet distillers’ grains as an additional feed to the existing dry grind plants for increased ethanol yield requires a different approach to the compositional analysis of the material. Rather than focusing on its nutritional value, this new approach aims at determining more detailed chemical composition, especially on polymeric sugars such as cellulose, starch, and xylan, which release fermentable sugars upon enzymatic hydrolysis. In this paper, we present a detailed and complete compositional analysis procedure suggested for DDGS and wet distillers’ grains, as well as the resulting compositions completed by three different research groups. Polymeric sugars, crude protein, crude oil, and ash contents of DDGS and wet distillers’ grains were accurately and reproducibly determined by the compositional analysis procedure described in this paper.