The impedance transition and electron series resonance at an RF bias substrate were observed in 13.56 MHz inductively coupled plasma (ICP). As ICP coil power increased, the impedance of the RF bias transitioned from a capacitive to an inductive load. When bias voltages and discharge impedances reached minimums, bias voltages and currents were in-phase during the transition. The transition can be understood as a series LC resonance between the sheaths (capacitor) and plasma bulks (inductance due to electron inertia). This corresponds to the electron series resonance (ESR) observed in very high-frequency capacitive discharges, and a new ESR frequency which considers sheath resistances is presented. (C) 2010 Elsevier B.V. All rights reserved.