Investigations of the thermodynamic and kinetic properties of the Co 2 + , Ni 2 + , Cu 2 + , Zn 2 + , Cd 2 + and Pb 2 + complexes ofN,N ,N ,N -tetrakis(2-hydroxyethyl)-1,4,7,10-tetraazacyclododecane (THEC-12) and the newly synthesisedN,N ,N ,N -tetrakis(2-hydroxyethyl)-1,4,8,12-tetraazacyclopentadecane (THEC-15) have been undertaken. Formation constants for the ML 2 + complexes indicate that the hydroxyethyl groups cause significant destabilisation of the complexes with respect to those of the non-hydroxyethylated macrocycles. For decomplexation of [M(THEC-12)] 2 + in aqueous HNO 3 the observed rate constants = k o b s = k 0 + k H [H + ], where at 298.2 K, k 0 = (3.06 ± 1.86) 10 - 5 s - 1 when M = Co, k 0 0 s - 1 when M = Ni and Cu, and 10 5 k H = 61.4 ± 2.9, 5.78 ± 1.42 and 2.81 ± 0.09 dm 3 mol - 1 s - 1 when M = Co, Ni and Cu, respectively. For [M(THEC-15)] 2 + , decomplexation is very slow with t 1 2 > 24 h (in 1.00 mol dm - 3 HNO 3 ) when M = Co and Ni, but considerably faster witht 1 2 18 s (1.00 mol dm - 3 HNO 3 ) when M = Cu.