  The CeIn3-xSnx cubic heavy fermion system presents an antiferromagnetic
transition at T_N = 10 K, for x = 0, that decreases continuously down to 0 K
upon Sn substitution at a critical concentration of x_c ~ 0.65. In the vicinity
of T_N -> 0 the system shows non-Fermi liquid behavior due to antiferromagnetic
critical fluctuations. For a high Sn content, x > 2.2, intermediate valence
effects are present. In this work we show that Gd3+-doped electron spin
resonance (ESR) probes a change in the character of the Ce 4f electron, as a
function of Sn substitution. The Gd3+ ESR results indicate a transition of the
Ce 4f spin behavior from localized to itinerant. Near the quantum critical
point, on the antiferromagnetic side of the magnetic phase diagram, both
localized and itinerant behaviors coexist.
