High-density magnetic-field-free plasma sources produce plasmas that are opaque to radio frequency (rf) fields in the 0-200 MHz frequency range. Thus plasma currents from a biased substrate flow to ground along reactor surfaces. We investigate some consequences of this rf skin effect in an inductively coupled plasma source with densities of 10(11)-10(12)cm(-3) in argon. Magnetic probe measurements confirm that capacitively coupled rf fields are localized near the reactor surfaces. Electric probes were used to measure the voltage on the surface of a biased platen without a substrate. We find that the rf wavelength and phase velocity along reactor surfaces are reduced by a factor of similar to 5 compared to free space. This reduced wavelength is attributed to a surface wave which can be analyzed using a formalism similar to that for Trivelpiece-Gould modes.