An Inductively Coupled Plasma source was previously applied by us to surface treatment of a plastic material in moderate power conditions. In these conditions the coupling between the antenna and the plasma is capacitive and is usually referred to as E-mode. The machine has been modified to operate at working powers up to1 kW, thus providing a real inductive coupling, referred to as H-mode. The plasma is generated by applying a 13.56 MHz radio-frequency to a /4 antenna wrapped outside a quartz tube. The plasma chamber is evacuated by a rotary pump down to about 0.1 Pa and eventually fed with Argon at working pressures in the range from 1 to 100 Pa. In the present work the transition from capacitive to inductive coupling mode is investigated by means of electrical and optical methods. Experiments reveal that the transition occurs at decreasing power levels, with increasing pressure in the above mentioned range. The percentage of power that is delivered to the plasma is investigated, too, and appears to be proportional to the input power

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