Three high-nuclearity mixed valence manganese(II/III) coordination clusters, have been synthesised, that is, [Mn(III) (6)Mn(II) (4)(μ(3)-O)(4)(HL(1))(6)(μ(3)-N(3))(3)(μ(3)-Br)(Br)](N(3))(0.7)/(Br)(0.3)⋅3 MeCN⋅2 MeOH (1) (H(3)L(1)=3-methylpentan-1,3,5-triol), [Mn(III) (11)Mn(II) (6)(μ(4)-O)(8)(μ(3)-Cl)(4)(μ,μ(3)-O(2)CMe)(2)(μ,μ-L(2))(10)Cl(2.34)(O(2)CMe)(0.66)(py)(3)(MeCN)(2)]⋅7 MeCN (2) (H(2)L(2)=2,2-dimethyl-1,3-propanediol and py is pyridine), and [Mn(III) (12)Mn(II) (7)(μ(4)-O)(8)(μ(3)-η(1)N(3))(8)(HL(3))(12)(MeCN)(6)]Cl(2)⋅10 MeOH ⋅MeCN (3) (H(3)L(3)=2,6-bis(hydroxymethyl)-4-methylphenol) with high ground-spin states, S=22, 28±1, and 83/2, respectively; their magnetothermal properties have been studied. The three compounds are based on a common supertetrahedral building block as seen in the Mn(10) cluster. This fundamental magnetic unit is made up of a tetrahedron of Mn(II) ions with six Mn(III) ions placed midway along each edge giving an inscribed octahedron. Thus, the fundamental building unit as represented by compound 1 can be described as a Mn(10) supertetrahedron. Compounds 2 and 3 correspond to two such units joined by a common edge or vertex, respectively, resulting in Mn(17) and Mn(19) coordination clusters. Magnetothermal studies reveal that all three compounds show interesting long-range magnetic ordering at low temperature, originating from negligible magnetic anisotropy of the compounds; compound 2 shows the largest magnetocaloric effect among the three compounds. This is as expected and can be attributed to the presence of a small magnetic anisotropy, and low-lying excited states in compound 2.