Nanomaterials with high specific surface area and high absorption capacity are attracting increased interest aimed at imparting the desired magnetic properties. This work is devoted to the study of the effect of heat treatment in a hydrogen atmosphere on the microstructure, adsorption and magnetic properties of heterogeneous FePt/h-BN nanomaterials. Obtained via the polyol process, FePt nanoparticles (NPs) had a size < 2 nm and were uniformly distributed over the surface of hexagonal boron nitride (h-BN) nanosheets. The temperature-activated fcc→fct phase transformation in ultrafine FePt NPs has been well documented. FePt NPs act as active centers dissociating H₂ molecules and transfer adsorbed hydrogen atoms to the h-BN. Density functional theory (DFT) calculations also indicate that the h-BN substrate can absorb hydrogen adsorbed on the FePt NPs. This hydrogen circulation in the FePt/h-BN system promoted the fcc→fct phase transformation and allowed to control the magnetic properties. FePt/h-BN nanomaterials also exhibited a high adsorption capacity with respect to various organic dyes.