An Inductively Coupled Plasma (ICP) exhibits two distinct modes of operation. A low input power capacitive E-mode, and a high input power inductive H-mode. The gas initially breaks down m the E-mode, switching to H-mode as input power is increased above a certain threshold. This transition between the E and H modes is observed by a dramatic increase in light output from the plasma, and a ‘glitch3 in the antenna current as the load characteristics of the plasma change from capacitive to inductive. The transition between the E and H modes exhibits hysteresis. The effect of introducing small amounts of molecular oxygen in an argon plasma on the E-to-H transition has been investigated. It has been observed that the addition of small amounts of molecular gas increases the size of the hysteresis instability window in the system, and also increases the ignition current required for the inductive mode. Withm the E-to-H instability window, gross spatial mhomogeneities (plasmoids) have been observed. We report the observation of plasmoids in a cylindrical inductively coupled rf plasma. Observations show plasmoid lobes as regions of higher plasma density and light emission distributed symmetrically around the antenna. They may also rotate and the number of lobes may change depending of plasma power, pressure, and gas mixture. In order to explain the behaviour of plasmoids we present the discharge as plasma transmission line with associated line capacitance and inductance as defined by the plasma. Plasmoids may be shown to be standing waves showing density and absorbed power modulation consistent with observed experimental results. A plasma simulation coupled to the theoretical model also shows results consistent with experimental observation.