Differently structured aluminum (tri/mono) hydroxide (Al(OH) 3 /AlO(OH)) nanoparticles were prepared and used as thermal-management additives to microfibrillated cellulose (MFC), cast-dried in thin-layer films. Both particles increased the thermal stability of the MFC film, yielding 20–23% residue at 600 °C, and up to 57% lowered enthalpy (to 5.5–7.5 kJ/g) at 0.15 wt% of loading, while transforming to alumina (Al 2 O 3 ). However, the film containing 40 nm large Al(OH) 3 particles decomposed in a one-step process, and released up to 20% more energy between 300 and 400 °C as compared to the films prepared from smaller (21 nm) and meta-stable AlO(OH), which decomposed gradually with an exothermic peak shifted to 480 °C. The latter resulted in a highly flexible, optically transparent (95%), and mechanically stronger (5.7 GPa) film with a much lower specific heat capacity (0.31–0.28 J/gK compared to 0.68–0.89 J/gK for MFC-Al(OH) 3 and 0.87–1.26 for MFC films), which rendered it as an effective heat-dissipating material to be used in flexible opto-electronics. Low oxygen permeability (2192.8 cm 3 /m 2 day) and a hydrophobic surface (> 60°) also rendered such a film useful in ecologically-benign and thermosensitive packaging.