With their nanoscalar, superparamagnetic Gd3+-ion clusters (1 ´ 5 nm) confined within ultrashort (20-80 nm) single- walled carbon nanotube capsules, gadonanotubes are high- performance T1-weighted contrast agents for magnetic resonance imaging (MRI). At 1.5 T, 37 °C, and pH 6.5, the r1 relaxivity (ca. 180 mM-1 s-1 per Gd3+ ion) of gadonanotubes is 40 times greater than any current Gd3+ ion- based clinical agent. Herein, we report that gadonanotubes are also ultrasensitive pH-smart probes with their r1/pH response from pH 7.0-7.4 being an order of magnitude greater than for any other MR contrast agent. This result suggests that gadonanotubes might be excellent candidates for the development of clinical agents for the early detection of cancer where the extracellular pH of tumors can drop to pH ) 7 or below. In the present study, gadonanotubes have also been shown to maintain their integrity when challenged ex vivo by phosphate-buffered saline solution, serum, heat, and pH cycling.