Handling of material in nuclear fuel cycle treatments requires special facilities and considerations. One of the problems is the need for remote monitoring and control of the constituents during selected treatment process, e.g. solvent extraction. The Plutonium-Uranium Extraction (PUREX) Process has been used extensively for extraction of uranium and plutonium from used nuclear fuel and is based on tributyl phosphate dissolved in an aliphatic solvent, kerosene or similar. Commercial reprocessing plants in a number of countries, e.g. France, England and Japan, have successfully used PUREX for several decades. However, some problems requiring careful control still exist. For example, it has been shown that this system will extract large amounts of nitric acid, which can lead to formation of aggregates in the organic phase and eventual appearance of a third phase. This study is focused on the possibility of using Near-IR measurements to detect and quantify nitric acid dissolved in organic phase during a process, i.e. on-line measurements, and the possibility to predict and avoid third phase formation. Spectra for the organic phase were collected and shifts observed in the spectra can be related to nitric acid and water uptake.