Molecular beam epitaxial growth of Zn1-xMgxS alloy thin films on GaP (1 0 0) substrates is reported. In situ reflection high energy electron diffraction (RHEED) studies show that the alloys can be grown with stable zinc-blende structure up to x around 30%. For x > 30% a phase transition will occur at a critical thickness which is sensitively dependent on the composition x. Several Schottky barrier photodetectors using Zn1-xMgxS layer, with thickness less than the critical thickness, as active layer were Fabricated. High ultra-violet responsivity and excellent visible rejection are achieved. The response curve of the Zn0.43Mg0.57S device offers a long wavelength cut-off at 295 nm and closely matches the erythemal action spectrum that describes human skill sensitivity to UV radiation.