The preparation and structural characterisation of five 3d-4f mixed metal phosphonate cages with general formula [Cr(III)6Ln(III)2(mu(3)-O)(2)(H2O)(2)((O3PBu)-Bu-t)(4)((O2CBu)-Bu-t)(12) (HO'Bu)(2)('PrNH2)(2)] where Ln(III) = La, 1; Tb, 3; Dy, 4; Ho, 5 and [(Cr6Gd2III)-Gd-III(mu(3)-O)(2)(H2O)(2)((O3PBu)-Bu-t)(4)((O2CBu)-Bu-t)(1 2)(HO'Bu)(4)] (2) are reported. The structure contains two oxo-centred (Cr-3) triangles, bridged by phosphonates and lanthanides. The magnetic behaviour of 1 has been modelled as two non-interacting isosceles triangles, involving two antiferromagnetic interactions (J(1) = -8.8 cm(-1)) with a smaller ferromagnetic interaction for the unique edge of the triangle (J(2) = +1.3 cm(-1)) giving an isolated S = 3/2 ground state per triangle. The quartet ground state has been proven through simulation of electron paramagnetic resonance (EPR) spectra obtained at the X- and Q-band. EPR simulations have also resulted in the introduction of small single-ion Zero Field Splitting (ZFS) parameters D = +/- 0.19 cm(-1) and rhombic term E = +/- 0.02 cm(-1), which are consistent with strong exchange limit calculations for an isolated S = 3/2 (D = +/- 0.22 and E = +/- 0.018 cm(-1)). ; Times Cited: 0