In this work, the permittivity of porous low-temperature cofired ceramics (LTCC) DP 951 is measured and evaluated. Porosification is performed at originally dense LTCC substrates in the fired state by a wet chemical etching procedure using hot phosphoric acid. Choosing this approach, areas with tailored permittivity can be generated in one single LTCC layer. The etch time and the bath temperature precisely control the penetration, and hence, the porosification depth. Therefore, the decrease in dielectric constant of the LTCC substrate can be correlated to the thickness of the porous layer. The dielectric constant is measured using a ring resonator in the microstrip configuration. From the resonances occurring in the transmission S-parameter vertical bar S(21)vertical bar spectrum between 1 and 10 GHz, the relative dielectric constant can be determined. Using 820-mu m-thick substrates, a relatively low reduction from epsilon(r) = 7.8 to 6.45 is achieved when a porosification depth of about 35 mu m is reached. Applying a fitting procedure, the dielectric constant of the pure porous layer is deduced to epsilon(r) = 2.3. Based on numerical simulations, the effective dielectric constant for a 100-mu m-thick glass-ceramic layer, which is modified to a depth of 35 mu m is calculated to 5.2, whereas the thickness represents a lower limit for commercially available tapes being typically implemented into the fabrication process of monolithic LTCC systems with integrated metallization planes. Compared with commercially available low-K LTCC materials, this value is lower than all other commonly used tape systems.