The heats of solution or substitution, Delta H-s(degrees), of RbI and CsI in TII crystal are reported, viz, -191.1 and -209.7 kJ mol(-1), respectively, along with the recovered lattice energies, Delta H-L(degrees), viz, -29.5 and -49.5 kJ mol(-1), respectively, and compared with some mixed alkali iodides. The large negative Delta H-L(degrees) values are far removed from those of their mixed alkali iodides, while the Delta H-L(degrees) values show reasonable consistency.