NaY samples containing Co as well as Mo are characterised by Mossbauer emission spectroscopy (MES), extended X-ray absorption fine structure (EXAFS) and TEM. After ion-exchanging Co and dehydrating the obtained CoNaY (the Co2+ ions migrate into the zeolite cation positions) two MO(CO)(6) molecules (average) are deposited inside each supercage. MES, EXAFS and TEM measurements show that very small 'Co-sulfide' particles are formed inside CoMoNaY after sulfidation. A large part of the cobalt exhibits an interaction with the NaY-lattice. According to EXAFS and TEM, small 'Mo-sulfide' species are formed inside the NaY supercages after stepwise sulfidation upto 773 K of CoMoNaY. These species consist of two Mo atoms with RMo-Mo approximate to 3.16 Angstrom (like in MoS2) surrounded by S and denoted by 'Mo(3.16 Angstrom)S-x'. The formation of these dimer species inside the zeolite supercages takes place via the intermediate formation of dimer 'Mo(2.80 Angstrom)S-x' species with RMo-Mo approximate to 2.80 Angstrom (like in MoS3). Although the 'Mo-sulfide' as well as the 'Co-sulfide' particles are present inside the zeolite cages no CoMoS formation (direct interaction between the Mo and Co) is observed. It is concluded that Mo deposited inside NaY does not fully prevent the formed 'Co-sulfide' particles in ion-exchanged CoNaY from decomposing into Co2+ and H2S. However, the 'Co-sulfide' particles are influenced to a lesser extent by a gas or temperature treatment in CoMoNaY than in CoNaY. The protolysis reaction which is responsible for the decomposition of 'Co-sulfide' particles in ion-exchanged CoNaY, causes in ion-exchanged CoMoNaY only a partial decomposition of the 'Co-sulfide' particles into Co2+ (located at NaY cation positions) and H2S both in the absence as well as in the presence of (HS)-S-2.