Amorphous YCo3films are contaminated by oxygen along a thin surface layer when exposed to air at room temperature. The field for resonance corresponding to this surface layer presents a dynamical shift with a well defined minimum. On the other hand linewidth presents a large maximum &Dgr;Hmaxas a function of temperature. After annealing in vacuum &Dgr;Hmaxdecreases and the dynamical shift is no more detected. The results obtained on a 500 A˚ thick sample are analysed quantitatively using formulas developed for the longitudinal relaxation mechanism. It is found that the results on both as deposited and vacuum annealed samples are well explained if we assume that the percentage of Co atoms in the ionic state is reduced to about one third of its value after the vacuum annealing.