The evolution of solid particles during crystallization frequently makes it impossible to measure concentration profiles by conventional sampling. Concentration profiles of the liquid phase can still be measured by coupling an ATR deep-immersion probe with a FTIR spectrometer. Using this system concentration profiles of azelaic acid initially dissolved in acetone could be obtained. Cooling is accomplished by directly injecting liquid carbon dioxide into the reaction mixture which causes the mixture to be a three-phase system. Although the system is highly turbulent, CO2 gas bubbles and evolving azelaic acid particles were not observed to influence the IR signal. Four different cooling rates of 4.2, 11.7, 18.5, and 26.1 degreesC/min were investigated. Concentration data points are obtained every five seconds giving a total of up to one hundred data points depending on the experimental settings. The FTIR absorption spectra are converted to volume-based concentration profiles by an interpolation type algorithm. Further conversion to mass-based concentration profiles made it possible to track the exact starting time of the crystallization process. It also eliminates the effect of decreasing temperature on the concentration profiles. By a comparison of all four experiments the detection of sub-cooling with increasing cooling rates is possible.