The NMR relaxivities of the decatungstolanthanoate core-shell nanoparticles, prepared by encapsulating [Ln(W₅O₁₈)₂]⁹⁻ polyoxometalates (LnPOM) within amorphous silica shells (K₉[Ln(W₅O₁₈)₂]@SiO₂), were studied along the Ln series. The relaxivity of GdPOM is slightly higher than for Gd-DTPA due to second-sphere relaxation effects, but the values for the other paramagnetic LnPOMs are much smaller due to the short T_1e values of their Ln³⁺-ions. The NPs have core-shell spherical structures, with LnPOM-containing cores with 9.5–28 nm diameters, and 4.0–11.0 nm thick amorphous silica shells. In water suspensions, the NPs have negative zeta potentials (−32.5 to −40.0 mV) and time-dependent hydrodynamic diameters (31–195 nm) reflecting the formation of aggregates. The relaxivities of GdPOM@SiO₂ NPs suspensions (r₁=10.97 (mM Gd)⁻¹ s⁻¹, r₂=12.02 (mM Gd)⁻¹ s⁻¹, 0.47 T, 25 °C) are considerably larger than for the GdPOM solutions, indicating that their silica shell is significantly porous to water. This increase is limited by the agglomeration of the complexes in the NPs core, limiting their access to water to those at the core surface. Replacing half of the Gd³⁺ ions by Eu³⁺ decreases the NPs r₁ and r₂ relaxivities at 0.47 T to 20 % and 35 % of their initial values, which are still considerable, but does not affect the efficient luminescence properties of the Eu³⁺ centers. This indicates that the mixed NPs have potential as dual modality MRI/optical imaging contrast agents.