This talk will focus on Carbon/Oxygen (C/O) logs acquired in complex formations that fall outside the normal range of reservoir conditions characterized by the service companies. C/O interpretation techniques will be reviewed, and examples in complex formations will be shown from: (a) Lost Hills, California [diatomite], (b) Kern River, California [three-phase fluid saturation conditions: steam/oil/water], and (c) Indonesia [high-temperature formations]. The examples serve to document what can be achieved in complex reservoir conditions that are less than ideal for C/O logging.