Near-infrared spectroscopy (NIRS) was used to measure changes of total hemoglobin concentration, 'Delta'(Hb)_(total), and relative vascular oxygenation 'Delta'(HbO2) in rat mammary and prostate tumors in response to hyperoxic gas interventions. 19F NMR of vascular perflubron was used to compare with the NIRS observations. The consistent trends between 'Delta'(Hb)_(total) and 'Delta'V_(T-blood) demonstrated that the NIRS can serve as an accurate, non-invasive, real time, monitoring tool for tumor vascular volume measurement. Meanwhile, these results also demonstrated that different types of tumors may respond to hyperoxic gases differently, and that NIRS could be used to assess vascular oxygenation changes non-invasively for guiding tumor therapy.