The infrared absorption in n-type InP was studied as a function of free-carrier concentration from 3.5×1017 to ∼ 1019 cm-3 in the spectral range of 10 μ to the fundamental absorption edge. The Burstein shifted edge, the normal intraband free-carrier absorption, and the interband free-carrier absorption were analyzed. The fundamental edge shifts to higher energy with increasing free-carrier concentration but only by 47% of the expected Burstein shift. The energy separation between the central-conduction-band minimum and the next higher conduction valleys in InP was found to be 0.90 ± 0.02 eV. © 1970 The American Physical Society.