The preparation and structural characterisation of five 3d-4f mixed metal phosphonate cages with general formula [Cr(III)6Ln(III)2(μ3-O)2(H2O)2(O3P(t)Bu)4(O2C(t)Bu)12(HO(i)Bu)2((i)PrNH2)2] where Ln(III) = La, 1; Tb, 3; Dy, 4; Ho, 5 and [Cr(III)6Gd(III)2(μ3-O)2(H2O)2(O3P(t)Bu)4(O2C(t)Bu)12(HO(i)Bu)4] (2) are reported. The structure contains two oxo-centred {Cr3} triangles, bridged by phosphonates and lanthanides. The magnetic behaviour of 1 has been modelled as two non-interacting isosceles triangles, involving two antiferromagnetic interactions (J1 = -8.8 cm(-1)) with a smaller ferromagnetic interaction for the unique edge of the triangle (J2 = +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)).