Alloy engineering offers a powerful route to tailor the optoelectronic properties of two-dimensional semiconductors. However, a comprehensive understanding of how alloy composition simultaneously affects the electronic structure, excitonic response, vibrational properties, and spin-valley physics remains limited. Here, we investigate monolayer ${\mathrm{MoS}}_{2x}{\mathrm{Se}}_{2(1-x)}$ alloys across the full composition range using optical spectroscopy. We demonstrate continuous tuning of the optical gap over $\sim 0.35\mathrm{eV}$, accompanied by a systematic reduction of the A-B 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 ${\mathrm{MoSe}}_2$ to $\sim 15\%$ in ${\mathrm{MoS}}_2$ 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 transition-metal-dichalcogenide monolayers.

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• Electronic structure
• Excitons
• Phonons
• Spin-orbit coupling
• Thermal properties
• Valley degrees of freedom
• First-principles calculations
• Photoluminescence