compounds with x = 0.0–2.0 have been investigated in detail. Substitution of Ge with Si leads to a monotonic decrease of both a and c along with concomitant contraction of the unit cell volume and significant modifications of the magnetic states-a crossover from ferromagnetism at room temperature for Ge-rich compounds to antiferromagnetism for Si-rich compounds. The magnetic phase diagram has been constructed over the full range of CeMn 2 Ge 2-x Si x compositions and coexistence of ferromagnetism and antiferromagnetism has been observed in CeMn 2 Ge 1.2 Si 0.8 , CeMn 2 Ge 1.0 Si 1.0 and CeMn 2 Ge 0.8 Si 1.2 with novel insight provided by high resolution neutron and X-ray synchrotron radiation studies. CeMn 2 Ge 2-x Si x compounds (x = 0, 0.4 and 0.8) exhibit moderate isothermal magnetic entropy accompanied with a second-order phase transition around room temperature. Analysis of critical behaviour in the vicinity of T C inter for CeMn 2 Ge 2 compound indicates behaviour consistent with three-dimensional Heisenberg model predictions. Due to the wide and interesting range of structural and magnetic phenomena-including magnetism, superconductivity, mixed valence, Kondo behaviour and heavy fermion-exhibited by layered RT 2 X 2 rare earth compounds, (R is a rare earth, T is transition metal and X is Si or Ge) this series has attracted significant attention over the years (e.g.