The results of a H-1 nuclear magnetic relaxation dispersion (NMRD) and EPR study on aqueous suspensions of Gd3+-loaded NaY nanozeolites and AlTUD-1 mesoporous material are described. Upon increase of the Si/Al ratio from 1.7 to 4.0 in the Gd3+-loaded zeolites, the relaxation rate per mM Gd3+ (r(1)) at 40 MHz and 25 degrees C increases from 14 to 27 s(-1) mM(-1). The NMRD and EPR data were fitted with a previously developed two-step model that considers the system as a concentrated aqueous solution of Gd3+ in the interior of the zeolite that is in exchange with the bulk water outside the zeolite. The results show that the observed increase in relaxivity can mainly be attributed to the residence lifetime of the water protons in the interior of the material, which decreased from 0.3 to 0.2 mu s, upon the increase of the Si/Al ratio. This can be explained by the decreased interaction of water with the zeolite walls as a result of the increased hydrophobicity. The importance of the exchange rate of water between the inside and the outside of the material was further demonstrated by the relatively high relaxivity (33 s(-1) mM(-1) at 40 MHz, 25 degrees C) observed for a suspension of the Gd3+-loaded mesoporous material AlTUD-1. Unfortunately, Gd3+ leaches rather easily from that material, but not from the Gd3+-loaded NaY zeolites, which may have potential as contrast agents for magnetic resonance imaging.