This chapter discusses excitonic optical properties and plasma effect of spontaneously ordered alloys. Some ternary semiconductor alloys of the type A1-yByC with zincblende-type structure show for y ≈ 0.5 a spontaneous ordering under suitable growth conditions in the sense that cation planes are periodically occupied with A and B atoms (or ions) in a direction . The material has then no longer cubic but uniaxial symmetry with point group C3v. This is the so called CuPt structure. Partial ordering can occur already for y ≈ 0.5 and trivially for y = 0.5. Evolution of a band gap and of a splitting of various valance bands is plotted as a function of a long range order parameter. The excitonic optical properties are treated in low- and medium-density regimes and the plasma effects are treated in a high-density regime.