The crystal structure of (Per) 2 [Co(mnt) 2 ], was investigated by single crystal X-ray diffraction. Its refinement shows a strong chemical dimerization of the anions [Co(mnt) 2 ] 2 2À . This is responsible for a unit cell doubling along the staking axis b when compared with other members of the a-(Per) 2 [M(mnt) 2 ] family of the low dimensional molecular conductors, with M ¼ Ni, Cu, Au, Pt or Pd. The dimerized [Co(mnt) 2 ] 2 2À stacks appear however as poorly correlated in the ac plane. The electrical conductivity of this compound measured in single crystals exhibits anomalous pressure dependence, where under increasing hydrostatic pressure the conductivity decreases while the metal insulator transition temperature is enhanced. This effect is ascribed to the dimeric nature of the counterions which imposes a sliding of the stacked perylene molecules along alternated directions in their planes.