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We present results for the complex refractive index and characteristic relative impedance of various commercially supplied solid absorbing materials at W-band (75-110 GHz). A waveguide measurement technique is described, together with the model that is fitted to the measured data. The accuracy of this method was verified with measurements of the well known dielectric material Rexolite 1422. Results are presented for Eccosorb MF-110, MF-114, MF-117, Ferrosorb, and TK-RAM. Knowledge of the electromagnetic properties of absorbing materials enables the design and manufacture of high performance waveguide or quasi-optical loads which find widespread application as thermal calibration targets and beam terminations in radar, spectroscopy, remote sensing, plasma diagnostics and astronomy.