A methyl ester of sinapic acid (MESA) has recently attracted attention due to its antioxidant action. This article presents results of a study on the spectral and physicochemical properties of MESA, using quantum chemistry (QC), steady-state (absorption and fluorescence), and time-resolved fluorescence techniques (TCSPC). The pKa of the phenol group in the ground state was determined (8.6). The pKa∗ values in the excited state calculated from the Förster cycle (1.9) and from fluorescence spectra (8.5) differed significantly but the experimental data suggested that the first was the more probable one. Quantum yields (QYs) for both forms have been determined. The QYs were very low (0.0017 and 0.0007) for nondissociated and dissociated forms, respectively and lifetimes were very short ≤10 ps for both forms. The differences in the probability of H-bond formation in the ground and the excited states were estimated by the application of the SdP polarity scale. Dipole moments in the ground state were calculated using QC. The ratio between dipole moments in the ground and the excited state for free molecule was obtained from Bilot-Kawski (B-K) method. Analysis of all collected results suggests that radical route (through hydrogen atom abstraction) of antioxidant activity of MESA is the more probable one in a water environment at pH below 6.