Theoretical results for the refractive index

of the ternary compounds Cd
xZn
1-xTe and Hg
xZn
1-xTe below the fundamental absorption edge are calculated as a function of photon energy and the mole fraction

. The theoretical result for

is obtained from a quantum mechanical calculation of the dielectric constant of a compound semiconductors. It is given in terms of basic material parameters only, with no adjustable constants. The reduction of the refractive index at the absorption edge as one increases

for Cd
xZn
1-xTe and for Hg
xZn
1-xTe as one decreases

is noticeable.