Transmission interference fringes from 2690 to 320 cm-1 for InAs and from 1330 to 320 cm-1 for CdTe have been analyzed with classical dispersion theory to obtain the room‐temperature dielectric constants. For InAs the dielectric constant between the band edge and the reststrahl ϵ∞ and the static dielectric constant ϵ0 are: ϵ∞ = 11.8±0.1, ϵ0 = 14.55±0.3; and for CdTe: ϵ∞ = 7.05±0.05, ϵ0 = 10.60±0.15. These values agree favorably with previously reported values. The absorption of InAs has been measured from 2500 cm-1, near the fundamental absorption edge, to 260 cm-1. Eight absorption peaks were observed between 444 and 269 cm-1 which are attributed to multiphonon combinations with the following characteristic phonon frequencies: TO1 = 222 cm-1, TO2 = 214 cm-1, LO = 196 cm-1, and LA = 143 cm-1. The transmission of CdTe has been observed from 10 000 to 220 cm-1. From 10 000 to 450 cm-1 the transmission is essentially constant at ∼60%. Below 400 cm-1 only two previously reported transmission minima were observed.