A novel amphiphilic GdPCTA-[12] derivative has been prepared. The complex formed micelles in aqueous solution with a relatively low CMC, 0.15 mM (25 °C). The concentration dependent T₁-relaxivity (r₁) of the system has been described. The maximum T₁-relaxivity, 29.2 s⁻¹ mM⁻¹ (20 MHz, 25 °C), was higher than for previously described micellar MRI contrast agents. This high T₁-relaxivity is a consequence of the favourable water residence time (τ_M) and the fact that the complex is heptadentate allowing two water molecules to coordinate to the gadolinium ion (q = 2).