Ni, Mo and the NiMo association were introduced in a stabilized HY zeolite. The modifications of the solids following sulfidation were examined by various techniques. We showed that metal sulfides are built up inside the zeolite. In most cases, sulfidation of the metals is incomplete. Full sulfidation is obtained only for the Ni zeolite processed with pure H2S and for the Mo zeolite when the Mo amount is very low. In the NiMo zeolite, Ni and Mo sulfidations are nearly additive. From XPS, Ni on the oxidized samples concentrates near the zeolite surface; it becomes better dispersed upon sulfidation. Mo is well dispersed in both oxidized and sulfided states. Ni dispersion is improved when combined with Mo. Zeolite OH bands connected with acidity were examined by FTIR. Introduction of either metal strongly decreases the Brönsted acidity. This acidity is regenerated upon sulfidation for the Ni and NiMo zeolites but not for the Mo zeolite. Changes in Lewis acidity are also observed.