Abstract Optically dark states play an important role in the electronic and optical properties of monolayers (MLs) of semiconducting transition metal dichalcogenides. The effect of temperature on the in-plane-field activation of the neutral and charged dark excitons is investigated in a WSe 2 ML encapsulated in hexagonal BN flakes. The brightening rates of the neutral dark ( X D ) and grey ( X G ) excitons and the negative dark trion ( T D ) differ substantially at particular temperature. More importantly, they weaken considerably by about 3–4 orders of magnitude with temperature increased from 4.2 K to 100 K. The quenching of the dark-related emissions is accompanied by the two-order-of-magnitude increase in the emissions of their neutral bright counterparts, i . e . neutral bright exciton ( X B ) and spin-singlet ( T S ) and spin-triplet ( T T ) negative trions, due to the thermal activations of dark states. Furthermore, the energy splittings between the dark X D and T D complexes and the corresponding bright X B , T S , and T T ones vary with temperature rises from 4.2 K to 100 K. This is explained in terms of the different exciton–phonon coupling for the bright and dark excitons stemming from their distinct symmetry properties.