We have used Raman spectroscopy to assess biomolecular changes of two human head and neck cancer xenografts in response to radiation therapy. We injected radiation-sensitive UM-SCC-22B and –resistant UM-SCC-47 cell lines into the flanks of 80 mice to grow tumor xenografts. Animals were distributed into control and radiation groups where the latter group received a single dose of 2 Gy. In vivo Raman spectroscopy was conducted before, 1, 24, and 48 hours after radiation and data were decomposed using principal component analysis and multi variate curve resolution. We found statistically significant differences in biomolecular changes among radiation-sensitive and –resistant groups.