Abstract: In a particular embodiment, a method and system are disclosed for measuring ion concentrations for a fluid and determining a degree of sample cleanup during sampling of a fluid downhole. The method includes but is not limited to deploying an ion selective sensor downhole, exposing the fluid to the ion selective sensor downhole, measuring ion concentrations of the fluid over time during sampling and estimating a degree of sample clean up from the ion concentration measurements. The system includes but is not limited to a tool deployed in a wellbore, an ion selective sensor in the tool, a processor in communication with the ion selective sensor and a memory for storing an output from the ion selective sensor.