An optical fiber reflectance probe is developed for fast, in situ detection of phase transitions between heterogeneous and homogeneous states in multiphase systems. Experiments with two binary systems, methanol-cyclohexane and carbon dioxide-methanol, are conducted to demonstrate the capability of the reflectance probe to capture the phase transitions in both liquid-liquid mixtures and gas expanded liquids (GXLs). The instantaneous revelation of transition in the latter from the gas-liquid two-phase state to a single phase at sufficiently high pressures provides important information for use and further investigation of this fully expanded regime. The current probe can withstand high pressures up to 100 bar and temperatures 100 degrees C, which suggests its applicability in many practical processes.