Near Infrared (NIR) spectroscopy has been demonstrated as a new means to quantify oxygenation of tissue vasculature, in particular changes in hemoglobin concentrations and oxygen saturation. A novel application such oximetry to monitoring of tumor vascular oxygenation during respiratory interventions will be presented. Our recent results obtained with a one-channel NIR spectrometer observing rat breast and prostate tumors in vivo have demonstrated a biphasic response of tumor vascular oxygenation when the inhaled gas is switched from air to carbogen. We present a mathematical model associating the tumor vascular oxygenation to both well-perfused and poorly perfused regions in the tumor. Furthermore, laboratory phantom experiments and computational simulations using the Finite Element Method have been conducted to verify this tumor hemodynamic model.