New isothiosemicarbazide-based heteronucleating ligands have been synthesised by Ba2+-templated (1:1) macrocyclisation of the polyetherdialdehydes, 3,3′-(3,6-dioxaoctane-1,8-diyldioxy)bis(2-hydroxybenzaldehyde) and 3,3′-(3,6,9-trioxaundecane-1,11-diyldioxy)bis(2-hydroxybenzaldehyde), by using S-methylisothiosemicarbazide as the diamine building block. Thus, the linear polyether dialdehydes (2-hydroxybenzaldehyde-3-yl)-O-(CH2CH2O)n-(3-yl-2-hydroxybenzaldehyde), with n = 3 and 4, react wit S-methylisothiosemicarbazide in the presence of Ba(CF3SO3)2 and Ni(CH3COO)2·4H2O to give two nickel/barium complexes 5 and 6, the crystal structures of which are described. 5: triclinic, P 1 , a=9.886(3), b=12.376(4), c=13.845(6) A , α=73.97(3), β=88.58(3), γ=83.26(3)°, Z=1 ; 6: triclinic, P 1 , a=10.767(2), b=13.221(2), c=15.986(2) A , α=102.57(1), β=101.13(1), γ=109.91(1)°, Z=2 . Treatment of these complexes with an aqueous solution of guanidinium sulfate yields the respective Ba free metallomacrocycles. The crystal structure of one of these (3·H2O) has been determined. 3·H2O: triclinic, P 1 , a=8.106(3), b=11.836(4), c=13.174(4) A , α=88.22(3), β=76.64(3), γ=70.30(3)°, Z = 2 . Its geometry is compared with its parent compound 5. It reveals that coordination of the Ba2+ cation (0.6 A above the mean plane of the O atoms) has little influence on the conformation of the macrocycle. The polyether ring in the two Ba complexes differs in size yet the Ba…Ni distances are remarkably similar (3.664(5) versus 3.723(3) A) suggesting that the two metal atoms although not bonding may be touching.