Using a simple modification of the classical electrical set-up, we perform electrochemical impedance measurements up to 1 MHz, which allow us to study electrosorption phenomena up to several A cm(-2) exchange current densities. In this way the usual electrosorption states on Pt(III), in sulphuric or perchloric solutions, are easily characterised. On the contrary unusual states involve extremely fast phenomena, the kinetics of which cannot be studied even up to I MHz: in sulphuric solution the whole process is extremely fast; in perchloric solution there is a multi-step phenomenon, but only one step can be kinetically characterised.