Microwave measurements of the properties of thin cross‐sectional elements of a dc hydrogen discharge have been made. These measurements were accomplished by exposure of portions of the discharge in the gap between the cones of an S‐band re‐entrant cavity operating in the TM010 mode. The equipment and techniques for making the microwave measurements employing the re‐entrant cavity are described. Measurements in the positive column of a dc hydrogen discharge showed the following results: Electron density increases with increasing tube current and also with increasing gas pressure in the pressure range measured (2 to 8‐mm Hg). Electron collision frequency is approximately equal to 4.6×109 multiplied by the gas pressure in mm Hg, and is independent of tube current. When measurements were made in the negative glow region, several anomalous effects including a negative frequency shift were observed. These have been explained qualitatively as being caused by plasma resonance phenomena. The fact that these plasma resonance phenomena were also observed at a frequency of 8550 Mc demonstrates that the electron density in portions of the negative glow is at least 300 times greater than the density in the positive column.