Using scanning tunneling spectroscopy (STS) and optical reflectance contrast measurements, we examine band-gap properties of single layers of transition metal dichalcogenide (TMDC) alloys: MoS 2 , Mo 0.5 W 0.5 S 2 , Mo 0.25 W 0.75 S 2 , Mo 0.1 W 0.9 S 2 , and WS 2 . The quasi-particle band gap, spin-orbit separation of the excitonic transitions at the K/K' point in the Brillouin zone, and binding energies of the A exciton are extracted from STS and optical data.The exciton binding energies change roughly linearly with tungsten concentration. For our samples on an insulating substrate, we report quasi-particle band gaps from 2.17 ± 0.04 eV (MoS 2 ) to 2.38 ± 0.06 eV (WS 2 ), with A exciton binding energies ranging from 310 to 420 meV.