One of the primary limiting factors in the estimation of water saturation from Carbon/Oxygen (C/O) ratios is the level of accuracy in the understanding and characterization of the wellbore environment. C/O logs are often run in complex wellbores that are outside the normal range of conditions characterized by the service companies. This paper shows examples of C/O logs from Kern River (California), Angola, and Nigeria that have been recorded in complex wellbore environments, such as: (a) through tubing-casing annuli, (b) large wellbores, (c) through gravel packs, (d) oil-contaminated observation wells, and (e) gas columns. Such environments are not yet characterized by the service companies. The examples serve to document what can be achieved in less than ideal C/O logging conditions.