Aqueous suspensions of lanthanide exchsanged zeolite LTL have been investigated by 1H, 17O NMR, and EPR relaxivity studies. Both the longitudinal and the transverse relaxivity of these Gd3+ loaded materials are strongly pH dependent and therefore, they have great potential as pH responsive contrast agents. For example, LTL-nanocrystals loaded with 3.5 wt % Gd show a dramatic decrease in the longitudinal relaxivity from 32 to 7 s(-1) mM(-1) (7.5 T and 25 degrees C) when going from pH 4 to 9. 1H and O-17 NMR show that this phenomenon can be rationalized by a decrease in proton mobility between the zeolite interior and the exterior due to a change from a fast prototropic exchange to a 3 orders of magnitude slower water exchange mechanism. The same material also has a high transverse relaxivity (98 s(-1) mM(-1) at 7.5 T, 25 degrees C, and pH 5 as measured with the CPMG pulse sequence), which is governed by proton exchange too, while water diffusion plays a minor role. The high relaxivities and pH dependence render Gd-loaded LTL materials promising pH responsive contrast agents. Since the r(2)/r(1) ratio of the designed probe strongly increases with the magnetic field strength, these materials are expected to be applicable for both T-1 and T-2 weighted imaging at low and high fields, respectively.