Refractive-index dispersion for the TlInGaAs quaternary alloys, grown on (100) InP substrates by gas-source molecular-beam epitaxy, has been measured at temperatures from 290 to 350 K in the photon-energy range of 1.2–2.0 eV by using spectroscopic ellipsometry. The temperature coefficient of the refractive-index decreases with increasing Tl composition. The result corresponds to the already reported reduced temperature dependence of the band-gap energy for the TlInGaAs because of the alloy of semiconductor InGaAs and semimetal TlAs. The result also agrees with the recently reported small temperature-variation of the lasing-wavelength for the TlInGaAs/InP laser diodes.