We report here on the optical response of elliptical gold particles arranged in a rectangular lattice on glass and probed at non-normal incidence in reflection using spectroscopic Mueller matrix ellipsometry in the energy range 0.73–5.9 eV. The surface plasmon resonances, reshaped near the Rayleigh anomalies, are mapped out by full azimuthal rotation of the sample. The experimental Mueller matrices are discussed, and interpretation is supported by finite element modeling of the Mueller–Jones matrix elements, which allows for identification of the dipolar and multipolar responses observed in the experimental spectra. The data show a strong polarization conversion around the surface plasmon resonances as a function of the azimuthal angle of incidence, also reshaped near the Rayleigh lines. An effective strong circular diattenuation is observed from the recorded Mueller matrix. We further argue the importance of Mueller matrix ellipsometry for metrology in the manufacture of metasurfaces, for understanding the effect of the lattice in metasurface design, and finally in validating computationalmethods.