When BN is synthesized via polymeric precursors and applied to ceramic substrates, tough adherent coatings of hexagonal-BN (h-BN) are obtained after annealing at 1200°C in N2. The study of these coatings is facilitated by using nonporous oxide powders containing single crystal particles of submicron size (e.g. cubes of MgO) as model ceramic substrates. These oxide powders permit high resolution TEM examination of the BN coatings with no further sample preparation. In this study, samples of BN/MgO cubes containing 50 wt% BN were heated in air at elevated temperatures for 16 hours to study the oxidation resistance of BN coatings. The BN coating was found to be stable at 600°C, but the 700°C-treated sample showed evidence for partial amorphization of the coating and reaction with MgO. A significant fraction of the MgO in the 800°C-treated sample had transformed to Mg2B2O5. The reaction of MgO with the BN coating under oxidizing ambients leads to loss of the cubic morphology in the precursor powder.