Two macrocyclic ligands, 1,4,7,10-tetraazacyclododecane-1-glutaric-4,7,10-triacetic acid (H5DOTAGA) and the novel 1,4,7,10-tetraazacyclododecane-1-(4-(carboxymethyl)benzoic)-4,7,10-triacetic acid (H5DOTABA), were prepared and their lanthanide complexes (Ln = Gd3+, Y3+) attached to an amino-functionalized T8-silsesquioxane. The novel compounds Gadoxane G (GG) and Gadoxane B (GB) possess eight monohydrated lanthanide complexes each, as evidenced by multinuclear (1H, 13C, 29Si) NMR spectroscopy and high resolution mass spectrometry (HR-MS). Pulsed-field gradient spin echo (PGSE) diffusion 1H NMR measurements revealed hydrodynamic radii of 1.44 nm and global rotational correlation times of about 3.35 ns for both compounds. With regard to potential MRI contrast agent applications, a variable-temperature 17O NMR and 1H nuclear magnetic relaxation dispersion (NMRD) study was carried out on aqueous solutions of the gadolinium(III) complexes of the Gadoxanes and the corresponding monomeric ligands to yield relevant physicochemical properties. The water exchange rates of the inner-sphere water molecules are all very similar (kex298 between (5.3 ± 0.5) × 106 s−1 and (5.9 ± 0.3) × 106 s−1) and only slightly higher than that reported for the gadolinium(III) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (H4DOTA) (kex298 = 4.1 × 106 s−1). Despite their almost identical size and their similar water exchange rates, GB shows a significantly higher longitudinal relaxivity than GG over nearly the whole range of magnetic fields (e.g., 17.1 mM−1 s−1 for GB and 12.1 mM−1 s−1 for GG at 20 MHz and 25 °C). This difference arises from their different local rotational correlation times (τlR298 = 240 ± 10 ps and 380 ± 20 ps, respectively), because of the higher rigidity of the phenyl ring of GB as compared to the ethylene spacer of GG. A crucial feature of these novel compounds is the lability of the silsesquioxane core in aqueous media. The hydrolysis of the Si−O−Si moieties was investigated by 29Si NMR and PGSE diffusion 1H NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), as well as by relaxivity measurements. Although frozen aqueous solutions (pH 7.0) of GG and GB can be stored at −28 °C for at least 10 months without any decomposition, with increasing temperature and pH the hydrolysis of the silsesquioxane core was observed (e.g., t1/2 = 15 h at pH 7.4 and 55 min at pH 8.1 for GG at 37 °C). No change in relaxivity was detected within the first 3 h, since the hydrolysis of the initial Si−O−Si moieties has no influence on the rotational correlation time. However, the further hydrolysis of the silsesquioxane core leads to smaller fragments and therefore to a decrease in relaxivity.