Peritoneal carcinomatosis from appendix or colon (large bowel) cancer is treated in suitable patients with surgery followed by instillation of heated chemotherapy inside the abdominal cavity. This procedure is termed 'Hyperthermic intraoperative Peritoneal Chemoperfusion' or HIPEC. Many center perform HIPEC with high dose oxaliplatin, a standard chemotherapy drug active against colon cancer, administered during 30 minutes at 41°C. The hypothesis of this study is, that chemoperfusion at normal (37.5°C) temperature but longer duration (90 minutes) may be safer and at least as efficient. Patients will be treated with one of three possible HIPEC regimens using oxaliplatin: high dose, 30 min, 41°C; high dose, 30 min, 37.5°C; or low dose, 90 min, 37.5°C. The outcome parameters are pharmacokinetic and pharmacodynamic: using specialized techniques, tissue penetration of chemotherapy and cancer cell kill effects will be compared in order to establish the safest and most active HIPEC regimen.