The specific crystallographic features of Ni1/3TiS2 and Ni1/4TiS2 led us to carry out ultrahigh vacuum-scanning tunneling microscopy (UHV-STM) studies, in order to bring to light the different atom chemical environments of the structure and the host-guest interactions. These compounds present specific structural reorganizations that involve, in particular, different chemical surroundings for chalcogen atoms (the outer layer of the compounds). We imaged the top sulfur planes (001) for both compounds and propose an explanation of the two well-differentiated contrasts observed. (C) 2000 Elsevier Science Ltd. All rights reserved.