The crystal structure of the R6Si4Se17 (RxA0;= La and Ce) selenides was studied by means of X-ray powder and single crystal diffraction. The magnetic behaviour of the Ce-based compound was determined through SQUID magnetometry. The two materials crystallize with the Ce6Si4S17 structure type (space group , Pearson symbol aP54). Their unit cells comprise as many as six inequivalent positions of the rare-earth atoms. The compound Ce6Si4Se17 exhibits Curie-Weiss paramagnetism with no hint at any magnetic ordering down to 1.72 K. The magnetic susceptibility data indicates that the Ce ions in this compound may have fluctuating valence character.