Molecular recognition events in which lanthanide(ii1) ( Ln3+) cation pairs are formed have been studied using the template condensation crystalline products (Ln, -,Eu,),L( NO,),*H,O and (Ln, -,Tb,),L-(NO,),*H,O (where H,L is the [2 + 21 macrocyclic Schiff base obtained from 2.6-diformyl-p-cresol and 3,6-dioxaoctane-l,8-diamine). The observed relationship between the concentrations of Eu3+ (or Tb3+) in the reaction mixture X,, or X,, (from neutron activation analyses) with corresponding concentrations in the crystalline products ( x ) suggests that formation of LnEuL( NO,),*H,O heteromolecules is more favourable than that of LnTbL( NO,),*H,O. In both cases the cation discrimination index, computed as the ratio of probabilities of Ln3+ incorporation into the crystalline heterolanthanide compounds, is in favour of the larger Ln3+ ion. However indiscriminate complexation of Nd3+ and Eu3+ in the (Nd,-,-Eu,),L( NO,),-H,O system is unusual and reflects the importance of co-operative heteropair effects.Luminescence decay dynamics of the (Sm, -,Eu,),L( NO,),*H,O and (Pr, -,Tb,),L( NO,),*H,O systems (0 < x < 1 ) reveal two microscopic environments for Eu3+ and Tb3+ which were attributed to homodinuclear molecules, Ln,L(N0,),-H20 (Ln = Eu or Tb) (slow component) and heterodinuclear molecules SmEuL(NO,),=H,O and PrTbL( NO,),-H,O (fast component). The luminescence decay rate constants for intramolecularly coupled Eu-Sm and Pr-Tb pairs are 8200 and 1 2 500 s-', which yield coupling constants (a) of 2.9 x and 4.7 x lo", m6 s-' respectively when dominant dipolar interactions are assumed. No exchange interactions are evident despite the presence of a phenolate linkage shared by the heteroatoms only z 4 A apart. The ratio of Eu-Eu to Eu-Sm 'cation pairing selectivity' constants of 1 : 1.5 (expected ratio for random pairing is 1 : 2) supports the intervention of molecular recognition processes favouring the homoto hetero-paired species in the ion pairing events leading to (Sm, -,Eu,),L( NO,),*H,O compounds.