First experimental results of electron temperature modulation experiments in plasmas characterized by strong and long-lasting electron and ion Internal Transport Barriers (ITB) have been obtained in JET using ICRH in the Mode Conversion scheme. The ITB is shown to be a well localized narrow layer with low heat diffusivity, characterized by sub-critical transport and loss of stiffness. In addition, results from cold pulse propagation experiments suggest a second order transition process for ITB formation and can be reproduced by non-linear fluid turbulence simulations.