Lignin composition in relation to its basic phenylpropanoid units, particularly the syringyl to guaiacyl (S/G) ratio, is an important property for biomass characterization and varies greatly as a function of species, genotype and environment. A rapid screening method is highly desirable to assess lignin composition in a large number of samples. We have developed a nondestructive and label‐free Fourier transform Raman (FT‐Raman) spectroscopic method that is capable of rapidly and reliably measuring the S/G ratio with minimal sample preparation. A variety of feedstocks, including hardwood (Eucalyptus globulus), softwood (Pinus radiata), herbaceous plants (Zea mays, Panicum virgatum, and Sorghum bicolor), and a model dicot (Arabidopsis thaliana) were measured using this technique and the corresponding S/G ratio was calculated after spectral deconvolution based on the S and G bands identified using a known library of model compounds. The results obtained using this technique were successfully validated by pyrolysis‐gas chromatography/mass spectrometry (pyro‐GC/MS). This technique holds significant promise in the rapid screening of engineered feedstocks as part of a comprehensive screening methodology that is correlated with biomass recalcitrance. Biotechnol. Bioeng. 2012; 109:647–656. © 2011 Wiley Periodicals, Inc.