The magnetoelectric effect of chromium sesquioxide Cr2O3 has been determined experimentally as a function of temperature. One measures the electric field-induced magnetization on Cr2O3 crystals or the magnetic field-induced polarization. From the magnetoelectric moduli of Cr2O3 we extract a four-dimensional relativistic invariant pseudoscalar alpha-tilde. It is temperature dependent and of the order of ~10−4/Z0, with Z0 as vacuum impedance. We show that the new pseudoscalar is odd under parity transformation and odd under time inversion. Moreover, alpha-tilde is for Cr2O3 what Tellegen's gyrator is for two port theory, the axion field for axion electrodynamics, and the PEMC (perfect electromagnetic conductor) for electrical engineering.