The basic ideas of electron spin resonance (ESR) for paramagnetic ions diluted in superconducting compounds are presented. The resolved powder spectra and the fine structure narrowing process are discussed for S-state using a crystal field hamiltonian (Xc) involving only the b° parameter. The experimental ESR spectra at 35 GHz and 60 GHz for ~1 at. percent Gd+3 and Eu+2 in MMo6Sg (M Pb, Sn) are simulated and interpreted. For both ions SKC is dominated by a negative b°- Two values of the exchange parameter (/) are obtained: one from the ion to electron relaxation rate (Sie), the other from the g-shift. This latter is positive for Gd, but negative for Eu. The two values of J are of the order of 10 meV for both ions. The decrease of Sie below the superconducting critical temperature (Tc), seen through the fine structure narrowing process, allows us to extract an energy gap parameter of 2A(0)/fcBTc =5. The results are discussed and compared with experimental and theoreti¬ cal data in literature.