Photoluminescence (PL) spectroscopy has proven to provide deep insights into the optoelectronic properties of monolayer MoS2. Herein, a corresponding study is conducted on the excitonic properties of mechanically exfoliated monolayer MoS₂ under multivariate physical and chemical stimuli. Specifically, midgap exciton states that originate from lattice defects are characterized and they are compared to existing models. Through statistical data analyses of substrate-, temperature-, and laser-power-dependent measurements, a PL enhancement is revealed through physisorption of water molecules of the controversially discussed excited-state A biexciton (A^XX'). In addition, analyses of monolayer MoS₂ on gold substrates show that surface roughness does not account for changes in doping level within the material. Also, a shift in the electron–phonon coupling properties that arises from thin films of water that are physisorbed on top of the samples is reported.