Three PEGylated derivatives of 6-amino-2-[4,7,10-tris(carboxymethyl)-1,4,7,10-tetrazacyclododec-1-yl]hexanoic acid (DOTA-AHA) with different molecular weights were prepared and characterized. Their Gd-III chelates were studied in aqueous solution through variable-temperature H-1 nuclear magnetic relaxation dispersion (NMRD) and O-17 NMR spectroscopy to determine their relaxivities and the parameters that govern them. The relaxivity varied from 5.1 to 6.5 mM(-1)s(-1) (37 degrees C and 60 MHz) as the molecular weight of the polyethylene glycol (PEG) chain increased and was slightly higher than that of the parent chelate Gd(DOTA-AHA), owing to a small contribution of a slow global rotation of the complexes. A variable-temperature H-1 NMR spectroscopy study of several Ln(III) chelates of DOTA-A(PEG(750))HA allowed the determination of the isomeric M/m ratio (M = square-antiprismatic isomer and m = twisted square-antiprismatic isomer; the latter presents a much faster water exchange), which was estimated to be ca. 1:0.2 for the Gd-III chelate, very close to that of Gd(DOTA). This explains why the PEGylated Gd-III chelate has a water rate exchange similar to that of Gd(DOTA). The predominance of the M isomer is a consequence of the bulky PEG moiety, which does not favor the stabilization of the m isomer in sterically crowded systems at the substituent site, in contrast to what happens with less packed asymmetrical DOTA-type chelates with substitution at one of the four acetate C() atoms.