Photodynamic therapy (PDT) employs a combination of photosensitizing chemical, light, and oxygen Knowledge of tissue optical properties, including absorption (μa) and reduce scattering coefficients (μs’), makes possible to derive blood oxygen saturation, light penetration depth, and drug concentration, which are important to ensure PDT treatment efficacy at the specific wavelengths. We have developed an absorption spectroscopy system to measure μa and μs’ in the spectral range 600-800nm using a contact linear probe with a source fiber and multiple source-detector separation distances less than 1 cm. The μa and μs’ were recovered based on diffusion approximations of the photon transport equation. We measured tissue optical properties among various organs of patients with intraperitoneal malignancies for an on-going Phase II PDT protocol. The results from 12 patients showed various effective penetration depth from site to site and from organ to organ. The percentage oxygen saturation (%StO2) are similar before and after PDT. Before PDT, meff (mean (standard deviation) (number of patients)) in cm-1 at 630nm are 2.4 (0.2) (12) in small bowel, 2.2(0.4) (9) in large bowel, 4.2(2.7) (7) in tumor, 3.3 (0.3) (10) in peritoneum, 2.7 (0.3) (11) in skin, and 10.1 (0.6) (10) in liver. %StO2 is 60-80% for most organs but 30-40% for tumor.