Aqueous dispersions of poly(methacrylic acid)-coated superparamagnetic iron oxide nanoparticles (PMAA@SPIONs) and nanoparticles obtained by adding a layer of double-hydrophilic cationic block polyelectrolyte poly(ethylene oxide)-block-poly(N-methyl-2-vinylpyridinium iodide) (PEO-QP2VP) on PMAA@SPIONs were studied by a combination of static and dynamic light scattering, SAXS, transmission electron microscopy and atomic force microscopy, probing the structure of the SPION aggregates on the lengthscale from 1 to 10$^3$ nm. Both SALS and AFM results indicate that adding a PEO-QP2VP layer to PMAA@SPIONs decreases the size of SPION aggregates formed in the dispersions. While TEM micrographs show that PEO-QP2VP@PMAA@SPION particles are less apt to form small clusters with the size of several tens nm compared to PMAA@SPION particles, the local clustering has no effect on the power law scattering behavior (I(q) ∼ q$^{−1.4}$) of the SPION dispersions at longer lengthscales (tens to hundreds nm), which reflects mainly polydispersity of the aggregates.