Soft ferromagnetic magnetite (Fe3O4)-gadolinium borate nanocomposites have been prepared via a facile hydrothermal approach by using Fe3O4, borax (BX) or boric acid (BA) and a simple Gd(III) salt. The products obtained by depositing GdBO3 over Fe3O4 nanoparticles were analyzed by powder X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), Transmission Electron Microscopy (TEM), Thermogravi-metric Analysis (TGA) and magnetization measurements. The XRD patterns of the nanocomposites showed a bi-phase pattern with reflections matching with those of Fe3O4 and GdBO3. The results showed that the experimental parameters e.g. using either BX or BA, uncoated- or polyethylene glycol (PEG) coated-Fe3O4 as the precursors, changing reactant concentrations and the pH, all influence the structure of the borate coating, coverage type, size and morphology of the nanocomposites and saturation magnetization values to some extent. The coating was in the form of vaterite-type orthoborate [Gd-3(B3O9)] with BA and triclinic-GdBO3 with BX, both having the Gd/B ratio of 1/1. PEGylated Fe(3)O(4 )appeared to be a preferable support for coating with the borate layer to obtain smaller size and uniform composite structures. The nanocomposites prepared with BX displayed typical TEM images where a bunch of spherical 20-40 nm size magnetite nanoparticles were surrounded with a gadolinium borate cloud while nearly cubic, pseudo core-shell nanocomposite particles of 100-150 nm size were obtained with BA at pH 9. This low-cost approach offers the advantages of simplicity, efficiency and allows for the preparation of controlled nanocomposite structures by carefully tuning the experimental conditions. (C) 2017 Elsevier B.V. All rights reserved.