In1-xTlxP is proposed as a promising material for infrared detectors. A number of key optical and structural properties are studied within local density-functional theory. In1-xTlxP at x=0.67 and In1-xTlxAs at x=0.15 are estimated to have a gap of 0.1 eV. Their binding energies are larger than that of InSb, and they are found to form stable zinc-blende alloys for all x. In1-xTl xP nearly lattice matches to InP, and offers the potential to integrate detector array and read-out circuit. © 1994 American Institute of Physics.