A preliminary study on the 1–4.5 kW power industrial scale 27.12 MHz rf lambda-resonator oxygen asher is presented. Contact probes of several types, including single Langmuir and flat wall probes, thermocouples, and optical emission spectroscopy, are mainly used to diagnose plasma in the inductive source area, downstream chamber and in the vicinity of wafers. Electron density in a 200 mm wafer asher at 2 kW rf power varies from2×1011in the plasma source to5×107 cm−3in a downstream chamber 5–10 mm from a wafer. The ion density exceeds the electron density 10–60 times. The plasma space potential varies in a range of 14–22 V, while the floating potential of the bulk plasma and wall surface varies from +9 to −17 V. The minimum surface floating potential of −17 V exists at the maximum of the rf voltage standing wave distributed along the full lambda inductor. The wafer surface floating potential is in the range of 3–5 V depending on the reactor configuration and is constant within ±1 V on the 200 mm wafer. Positive ion current density on the wafer and downstream chamber surface is less than 1 μA/cm−2. The typical resist ashing nonuniformity is ⩽2%–5% (range, not sigma) for both 200 and 300 mm ashers at about a 6–8 μ/min ashing rate.