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Chemical vapor deposition (CVD)-grown flakes of high-quality monolayers of WS 2 can be stabilized at elevated temperatures by encapsulation with several layer hexagonal boron nitride ( h- BN), but to different degrees in the presence of ambient air, flowing N 2 , and flowing forming gas (95% N 2 , 5% H 2 ). The best passivation of WS 2 at elevated temperature occurs for h -BN-covered samples with flowing N 2 (after heating to 873 K), as judged by optical microscopy and photoluminescence (PL) intensity after a heating/cooling cycle. Stability is worse for uncovered samples, but best with flowing forming gas. PL from trions, in addition to that from excitons, is seen for covered WS 2 only for forming gas, during cooling below ∼323 K; the trion has an estimated binding energy of ∼28 meV. It might occur because of doping level changes caused by charge defect generation by H 2 molecules diffusing between the h -BN and the SiO 2 /Si substrate. The decomposition of uncovered WS 2 flakes in air suggests a dissociation and chemisorption energy barrier of O 2 on the WS 2 surface of ∼1.6 eV. Fitting the high-temperature PL intensities in air gives a binding energy of a free exciton of ∼ 229 meV.