The Gd 3 Ga 5− x Al x O 12(0≤ x≤ 5) solid solution has been prepared using ceramic synthesis routes and the structural and magnetic properties were investigated using x-ray diffraction, magnetic susceptibility, χ, and isothermal magnetisation, M (H), measurements. Our results indicate a contraction of the unit cell and more significant antiferromagnetic interactions as x increases. Despite the decrease in the magnetic polarisation on the application of a field and the corresponding decrease in the change in the magnetic entropy, ΔS, we find that Gd 3 Al 5 O 12 has a significantly higher observed (17%) and theoretical (14%) ΔS per unit mass than Gd 3 Ga 5 O 12. The theoretical increase in ΔS per unit volume (7%) is offset by the increased antiferromagnetic interactions in Gd 3 Al 5 O 12. The differences in ΔS are driven by a decrease in both the mass and the density as Al ions replace Ga ions. These results highlight the importance of changes to the crystal structure when considering materials for solid state magnetic cooling.