In this article, we discuss techniques for electrically characterizing a plasma processing tool for the purpose of measuring the plasma impedance at the fundamental frequency. This was accomplished by building a "probe station" containing commercially available current and voltage probes which can be inserted into the radio frequency (rf) coaxial cable through which rf power is supplied to the driven electrode. We have applied our techniques to a specific tool, the Tegal modified MCR-1 etch system. We find that an essential part of the characterization is the measurement of the impedance of the device when the plasma is extinguished, which is in parallel with the plasma impedance. We find that this impedance is an inductance in series with a capacitance, similar to that found for the GEC reference cell; thus, this equivalent circuit may be universal although the element values will be machine dependent. Measurement of plasma impedance at harmonic frequencies is discussed. For the Tegal tool, we find that when the rf power is applied as a step function at t = 0, the plasma impedance exhibits a transient for both its resistance and reactance. The transient occurs during the first 4 min after rf power is applied, during which the resistance and reactance decrease by 10%-20%, and then finally achieve a value that thereafter is nearly invariant in time. We have ruled out contamination, temperature effects, and plasma chemistry as the source of the transient. The impedance transient correlates well with a similar transient that we observe in the density of water vapor molecules that are present in the chamber, and we speculate that there may be a correlation between the two. The transient should be of concern to processing engineers who may find processing parameters changing during the process time, which is usually comparable to 4 min.