The compounds Gd6Co1.67Si3 and Tb 6Co1.67Si3, recently reported to form in a Ce6Ni2Si3-derived hexagonal structure (space group: P63/m) and to order magnetically below 295 and 190K respectively, have been investigated by means of detailed magnetization (M) studies in the temperature interval 1.8-330K as a function of magnetic field (H). The points of emphasis are as follows. We observe multiple steps in the M(H) curve for the Tb compound at 1.8K while increasing H, but these steps do not appear in the reverse cycle of H. At higher temperatures, such steps are absent. However, this 'staircase' behavior of M(H) is not observed for the Gd compound at any temperature and the isothermal magnetization is not hysteretic, unlike the case for the Tb compound. From the M(H) data measured at close intervals of temperature, we have derived the isothermal entropy change (ΔS) and it is found that ΔS follows a theoretically predicted H2/3 dependence. © 2008 IOP Publishing Ltd.