According to the results of DTA and RFA the features of interaction of the components in the TlI(TlI₃,Tl₂I₃)–Se systems were investigated and their phase diagrams were constructed. It was determined that, the TlI–Se system is quasi-binary and forms a phase diagram with monotectic and eutectic equilibria. Tl₂I₃–Se and TlI₃–Se systems are non-quasi-binary due to the incongruently melting character of initial compounds but stable lower solidus. Crystallization along these polythermal sections is a multi-stage process consisting of subsequent mono- and nonvariant equilibria.