Thermal decomposition of transannular peroxide of anthracene (POA) (or 9,10-epidioxido anthracene) was studied by means of electron paramagnetic resonance spectroscopy (EPR) in the solid as well as in the liquid phases. Decomposition process proceeds via cleavage of the O-O bridge of the POA molecule, generating thus an alcoxy intermediate radical. Its concentration increases to a certain equilibrium stage during the time scale of the experiment. EPR spectra in the solid state were of the singlet type at the temperatures over 350 K, a doublet like anisotropic spectra were measured at the room temperature, both having g-value 2.0033. EPR spectrum from POA decomposed in benzene indicates four protons with higher (2a H = 0.305 mT, 2a H = 0.335 mT) and four protons with a lower (2a H = 0.075 mT, 2a H = 0.105 mT) splitting constants, corresponding well the radical expected after cleavage of O-O bridge.