Ultrashort single-walled carbon nanotubes loaded with gadolinium ions (gadonanotubes) have been previously shown to exhibit extremely high T-1-weighted relaxivities (> 100 mM(-1) s(-1)). To further examine the effect of nanoconfinement on the relaxivity of gadolinium-based contrast agents formagnetic resonance imaging, a series of ultrashort single-walled carbon nanotube (US-tube) materials internally loaded with gadolinium chelates have been prepared and studied. US-tubes were loaded with Gd(acac)(3)-2H(2)O, Gd(hfac)(3)-2H(2)O, and Gd(thd) 3 (acac= acetylacetone, hfac= hexafluoroacetylacetone, thd= tetramethylheptanedione). The longitudinal relaxivities of the prepared materials determined at 25 degrees C in a 1.5 T field were 103 mM(-1) s(-1) for Gd(acac) 3-2H2O@ US-tubes, 105 mM(-1) s(-1) for Gd(hfac) 3-2H(2)O@ US-tubes and 26 mM(-1) s(-1) for Gd (thd) 3@ US-tubes. Compared with the relaxivities obtained for the unloaded chelates (< 10 mM(-1) s(-1)) as well as accounting for the T-1 reduction observed for the empty US-tubes, the boost in relaxivity for chelate-loaded US-tubes is attributed to confinement within the nanotube and depends on the number of coordinated water molecules. Copyright (C) 2014 John Wiley & Sons, Ltd.