This paper describes the preparation of GaxIn1 - xAsyP1 - y compound semiconductors by a gas-source molecular beam epitaxy method. Controlling the composition of GaxIn1 - xAsyP1 - y is difficult because of the different incorporation efficiencies of As and P and the dissimilar dependence of these efficiencies on growth parameters. In general, under normal growth conditions any increase in x, or in growth rate, increases the phosphorus sticking probability relative to the arsenic one. Perhaps the most unexpected result obtained in these studies is the observation that for particular alloys the composition changes abruptly at a certain growth temperature. This effect is caused by a sudden change in the sticking coefficients of As and P, due to surface reconstruction. Another finding is a miscibility gap in GaInAsP that is lattice-matched to InP at concentrations which correspond to the band-gap wavelengths of about 1.3-1.6 μm. The miscibility gap causes unstable layer growth. It may only be avoided by introducing lattice strain in GaInAsP or by growing the material under conditions which are far from thermodynamical equilibrium. A variation of the band-gap energy (Egap) as a function of composition will also be studied. The band-gap energy is determined experimentally throughout the entire composition of quaternaries lattice-matched to GaAs and to InP. The final topic to be discussed is the possibility of using elemental carbon (graphite) as a p-type dopant. Preliminary experiments on a commercially available electron-beam heated graphite source will be presented. The hole concentrations of 1.4 × 1020 cm-3 and 5 × 1019 cm-3 have been demonstrated for GaAs and Ga0.47In0.53As-on-InP, respectively. The corresponding Hall mobilities were 30 cm2 Vs-1 and 20 cm2 Vs-1.