Carrier-type modulation is demonstrated in 2D transition metal dichalcogenides as n-type monolayer MoSe2 is converted to nondegenerate p-type monolayer Mo1-xWxSe2 through isoelectronic doping. Although the alloys are mesoscopically uniform, the p-type conduction in mono layer Mo1-xWxSe2 appears to originate from the upshift of the valence-band maximum toward the Fermi level at highly localized "W-rich" regions in the lattice.