Solubility is an essential thermodynamic property that affects the efficiency of the supercritical dyeing process. There are many methods for measuring solubility in supercritical carbon dioxide. However, for substances with low solubility, there is a significant difference between the various methods, and some methods require additional quantitative analysis or calculations. As an alternative, a UV–Vis spectrometer can be used to measure solubility with high discrimination even at low concentrations. In this study, a method for measuring solubility in supercritical carbon dioxide using an in-situ UV–Vis spectrometer has been established that requires no additional calibration or assumptions. The solubility measurements of the disperse dye were performed at a temperature of 353–413 K and pressure from 15.5 MPa to 25 MPa. In addition, the solubility of benzoic acid was measured at a temperature of 318 K and compared with various literature values to verify this method.