The measurements of x-ray bremsstrahlung spectra from electron cyclotron resonance (ECR) ion sources have shown in the past that the electrons may gain energy inside the plasma up to about 2 MeV. In 2000 a series of experiments carried out at INFN-LNS with the SERSE source operating at 28 GHz provided evidence that high energy x-rays were produced, as the heat load on the cryostat was increased because of their dissipation on the cold mass. This result shows that the knowledge of ECR heating is incomplete, because the so-called adiabatic limit was exceeded by many electrons. More recently similar measurements were carried out at IMP, Lanzhou, with the SECRAL source, and with the VENUS (Versatile ECR for Nuclear Science) source at LBNL, operating at 28 GHz with a better plasma confinement than SERSE. It will be shown hereinafter that the generation of energetic electrons (up to 2 MeV) is mainly related to the magnetic field gradient; also the charge state distribution depends on the magnetic structure, and its variation can be studied in terms of regime transition from gentle to strong gradients. For this purpose, numerical simulations of the ECR heating based on a single particle approach have been used.