We investigate monolayer MoS2xSe2(1−x) alloys across the full composition range using optical spectroscopy. We demonstrate continuous tuning of the optical gap over ∼0.35 eV, accompanied by a systematic reduction of the B–A exciton splitting, in agreement with density functional theory calculations. Temperature-dependent measurements reveal a progressive increase of the average phonon energy from Se-rich to S-rich alloys that follows a simple reduced-mass scaling model. Polarization-resolved spectroscopy further shows a monotonic increase of the circular polarization from nearly zero in MoSe2 to ∼15% in MoS2 at 78 K. The observed evolution of the polarization is attributed to alloy-induced modifications of the electronic structure that modify bright–dark exciton mixing and the associated valley depolarization. These findings establish alloy engineering as an effective strategy for controlling excitonic properties in TMD monolayers.