Lanthanide(III) complexes of the chloro substituted 18-membered tetraaza macrocycle, 14,29-dichloro-3,10,18,25-tetraazapenta-cyclo[25.3.1.1 12,16.0 4,9.0 19,24]dotriaconta-1(31),2,4,6,8,10,12,14,16(32),17,19,21,23,25,27,29-hexadecaene-31,32-diol (H 2L 4), and the bromo substituted 18-membered tetraaza macrocycle, 14,29-dibromo-3,10,18,25-tetraazapentacyclo[25.3.1.1 12,16.0 4,9.0 19,24]dotriaconta-1(31),2,4,6,8,10,12,14,16(32),17,19,21,23,25,27,29-hexadecaene-31,32-diol (H 2L 5), have been synthesized by the template condensation of 2,6-diformyl-4-chlorophenol and 2,6-diformyl-4-bromophenol with 1,2-diaminobenzene, respectively, in the presence of hydrated lanthanide(III) nitrates in acetonitrile. Discrete mononuclear lanthanide(III) complexes of the composition [Ln(H 2L 4)(NO 3)(CH 3CN)](NO 3) 2. nH 2O (Ln=Nd 3+-Tm 3+ and Y 3+, except Pm 3+; n=1 for Nd 3+ and Y 3+, n=2 for Sm 3+, Gd 3+, Tb 3+, Dy 3+, Ho 3+ and Tm 3+ and n=3 for Eu 3+ and Er 3+) and [Ln(H 2L 5)(NO 3)(CH 3CN)](NO 3) 2. nH 2O (Ln=Sm 3+-Tm 3+ and Y 3+; n=2 for Sm 3+, Dy 3+, Ho 3+ and Er 3+ and n=3 for Eu 3+, Gd 3+, Tb 3+, Tm 3+ and Y 3+) have been synthesized in 20–35% yield. The chloro and bromo substituents on the macrocyclic framework do not affect the template potential of the lanthanide(III) cations. The formation of the complexes of H 2L 4 and H 2L 5 with lanthanide(III) ions is due to the adaptability of these macrocycles to coordinate according to the geometrical requirements of the metal ions and to the steric demands of the exocyclic ligands. The macrocycles coordinate as neutral ligands and the phenolic protons remain intact. The magnetic moments of the complexes are very close to the van Vleck values of the free lanthanide(III) cations. The complexes are stable in methanol and DMF over a period of 1 month. The complexes exhibit high thermal stability and the coordinated macrocycles are stable up to 360°C.