We evaluate Photofrin-mediated photodynamic therapy (PDT) in a phase 2 clinical trial as an adjuvant to surgery to treat peritoneal carcinomatosis. We extract tissue optical [reduced scattering (μ_s ^′), absorption (μ_a), and attenuation coefficients (μ _eff )] and physiological [blood oxygen saturation (%S _t O ₂), total hemoglobin concentration (THC), and photosensitizer concentration (c _Photofrin )] properties in 12 patients using a diffuse reflectance instrument and algorithms based on the diffusion equation. Before PDT, in normal intraperitoneal tissues %S _t O ₂ and THC ranged between 32 to 100% and 19 to 263 μM, respectively; corresponding data from tumor tissues ranged between 11 to 44% and 61 to 224 μM. Tumor %S _t O ₂ is significantly lower than oxygenation of normal intraperitoneal tissues in the same patients. The mean (±standard error of mean) penetration depth (δ) in millimeters at 630 nm is 4.8(±0.6) for small bowel, 5.2 (±0.67) for large bowel, 3.39(±0.29) for peritoneum, 5.19(±1.4) for skin, 1.0(±0.1) for liver, and 3.02(±0.66) for tumor. c _Photofrin in micromolars is 4.9(±2.3) for small bowel, 4.8(±2.3) for large bowel, 3.0 (±1.0) for peritoneum, 2.5(±0.9) for skin, and 7.4(±2.8) for tumor. In all tissues examined, mean c _Photofrin tends to decrease after PDT, perhaps due to photobleaching. These results provide benchmark in-vivo tissue optical property data, and demonstrate the feasibility of in-situ measurements during clinical PDT treatments. © 2005 Society of Photo-Optical Instrumentation Engineers.