Accelerated research efforts to develop viable and economical solutions for the production of ethanol from lignocellulosic material has resulted in a need for fast, accurate, and inexpensive methods for compositional analyses of biomass used as process feedstock. One of the challenges faced by analytical chemists is the development of analytical methods for very rapidly measuring the composition of intermediate process streams generated from the dilute-acid pretreatment of biomass. This work documents the development of two rapid analytical techniques based on Near-Infrared (NIR) spectroscopy for the compositional analysis of pretreated corn stover slurries and associated liquors. The liquor method employs transmission NIR spectroscopy to measure the composition of dissolved constituents in pretreatment liquors. The slurry method employs transflectance NIR spectroscopy to measure the composition of the insoluble solids fraction in pretreatment slurries. The results of this effort have demonstrated that it is possible to obtain compositional data for dilute acid pretreated slurries and associated liquors without the need for time and labor intensive sample preparation steps (washing, drying, and milling) required by wet chemical and other NIR spectroscopic techniques. Compositional data for these process streams can now be made available to researchers in minutes rather than days, and could be used by industry to optimize process performance.