A convenient real-time monitoring of monomer concentration during living cationic ring-opening polymerizations of tetrahydrofuran (THF) initiated with methyl triflate (MeOTf) has been developed for kinetic investigation and determination of equilibrium monomer concentration ([M],e,) ,via in situ, FTIR spectroscopy in combination with a diamond tipped attenuated total reflectance (ATR) immersion probe. The polymerization rate was first order with respect to monomer concentration and initiator concentration from the linear slope of ln([M],0,−[M],e,)/([M]−[M],e,) versus polymerization time at different temperatures in various solvents. [M],e, decreased linearly with initial monomer concentration while increased exponentially with increasing polymerization temperature. The equilibrium also strongly depends on solvent polarity and its interaction with monomer. The equilibrium polymerization time (,t,e,) decreased with increasing solvent polarity and decreased linearly with increasing [M],0, in three solvents with different slopes to the same point of bulk polymerization in the absence of solvent. Equation of ,M,n,e, = 72.1/(0.14–0.04[M],e,) has been established to provide a simple and effective approach for the prediction for the number-average molecular weight of polyTHFs at equilibrium state (,M,n,e,) in the equilibrium living cationic ring-opening polymerization of THF at 0 °C.