Oxygenation of a tumor is one of the most important predictive factors: hypoxia is associated with aggressive tumors and substantially lower survival rate. Despite this high relevance of tumor oxygenation, there is currently no bedside technique available to measure it in clinical routine care. The aim of this work is to determine the oxygenation of tissue in mice by a continuous wave multispectral near-infrared optical tomograph (mNIROT). Tomographic reconstructions were processed by a massively modified NIRFAST software. We quantitatively measured the tissue oxygen saturation of the tumors in 4 BALB/c nude, female mice with human colon carcinoma cancer cells DLD-1 KRAS injected subcutaneously. The study revealed changes of oxygenation in tumors on the long-term.