Dielectric‐loaded accelerating structures offer the potential of a simple, inexpensive alternative to copper disk‐loaded structures for use in high‐gradient rf linear accelerators. However, a variety of potential problems must be studied in order to demonstrate this potential, including rf breakdown, joule heating, and gas desorption. A joint Argonne National Laboratory — Naval Research Laboratory program is under way to investigate these issues, using high‐power 11.424‐GHz radiation from the NRL Magnicon Facility. Recent tests using a copper accelerating tube with an alumina liner have shown no evidence of rf breakdown at up to 5 MW drive power (equivalent to 8 MV/m accelerating gradient). However, substantial absorption of the incident microwave radiation was observed, accompanied by visible light emission from the dielectric surface, indicating the presence of electron multipactor in the tube. © 2003 American Institute of Physics