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Measurements of dielectric constant and loss tangent have been made on several paraelectric and ferroelectric materials in the region of 35 Gc/s. The measurement technique is appropriate for materials with large dielectric constant and large loss tangent; typical values measured are in excess of 1500 for dielectric constant and 0.1 for loss tangent. Preliminary considerations indicate an accuracy of approximately 5 per cent. Previously reported techniques are not suitable for making such measurements at 35 Gc/s, and this aspect is discussed briefly. The technique consists of measuring insertion loss and phase shift of a signal upon passing through a slab of the test material filling the cross section of the waveguide. These data are measured as a function of slab thickness. The insertion loss (in dB) will undulate with increasing thickness for sufficiently thin samples, but it quickly becomes proportional to thickness for material having the afore-mentioned properties. In the region of proportionality the equations relating insertion loss and phase shift to the dielectric constant and loss tangent become rather simple, allowing data to be reduced easily. The paper gives a derivation of the equations required for data reduction and also reports measured dielectric constants and loss tangents of various barium-strontium titanates at 35 Gc/s. Experimental problems are discussed including the very real problem of mounting the sample within the waveguide so as to eliminate air gaps and erratic data.