Under stoichiometric conditions, the segmental tris-tridentate ligand L9 assembles with two different lanthanide metal ions Ln(1) and Ln(2) (Ln(1), Ln(2)=La, Nd, Sm, Eu, Yb, Lu, Y) to give mixtures of the heterotrimetallic triple-stranded helicates [(Ln(1))(x)(Ln(2))(3-x)(L9)(3)](9+) (x=0-3) in acetonitrile. The combination of qualitative (ESI-MS) and quantitative ((1)H NMR) speciations provides a set of thermodynamic data that were analysed with various statistical chemical models. A satisfying description requires the consideration of different affinities for the terminal N(6)O(3) sites (k(t)(Ln) and for the central N(9) site (k(c)(Ln) for each specific lanthanide. The nontrivial dependence of these parameters on the ionic radius provides size-discriminating effects that favour the formation of heterotrimetallic helicates in which the central site is occupied by the larger metal of the pair. Combining the latter enthalpic driving forces with entropic contributions due to specific stoichiometric conditions allows partial selection (i.e., programming) of a specific heterotrimetallic species in solution, which can be isolated by crystallisation, as demonstrated for [Eu(2.04)La(0.96)(L9)(3)](CF(3)SO(3))(9)(CH(3)NO(2))(9) (1, Eu(2.04)La(0.96)C(207)H(222)N(48)O(51)S(9)F(27), monoclinic, P2(1)/c, Z=4) in which the cation [EuLaEu(L9)(3)](9+) is the major component in the crystal. The scope and limitation of this approach is discussed together with the conditions for explicitly considering intermetallic interaction parameters u(Ln1Ln2) in more sophisticated chemical models.