The purpose of this article is to provide a genealogical and prosopographical study of the epi tou kanikleiou Alexios Palaiologos Tzamblakon ( Ἀλέξιος Παλαιολόγος Τζαμπλάκων), who was the father-in-law of the 15th-century Byzantine historian Georgios Sphrantzis. The Tzamblakones had family relations with the Palaiologos dynasty and the Genose Zaccaria family, but the details still remain to be clarified. A reinterpretation of some contemporary sources suggests that the Tzamblakones were not descended from the Zaccaria family, but rather from a Zuan Blacho-Bamblachus of Vlach origin, known from the Latin sources.<br>　The Chronikon of Sphrantzis mentions a "wife's uncle (γυναικοθεῖος)" - Tzamblakon Kydonidis in the Peloponnese (XXXIX. 5). And there is a document that mentions a Joannes Zamblaco, "the cousin" of the despotis Thomas Palaiologos (1430-1460). These records suggest that Kydonidis was not a relative of Thomas's wife as some scholars have stated, but of Sphrantzis's wife Eleni. Family relations between the despot and the Tzamblakones can be traced back to the megas stratopedarchis Dimitrios Tzamblakon, who was husband of Evdokia, the great-granddaughter of the emperor Michail VIII (1259-1282). Alexios, the only known Palaiologos-Tzamblakon, is supposed to be their descendant (see the genealogical table).<br>　The Sphrantzai and the Tzamblakones had a common social background, and marriage relations between them mutually strengthened their political positions. Sphrantzis himself became a relative of the imperial family and gained a political position in the Peloponnese, where the Tzamblakones had an estate. On the other hand, the Tzamblakones, who were in political obscurity, regained their position at the court via their connection with Sphrantzis, who was a favorite of the emperor. Moreover, it is also attested that Sphrantzis sought refuge for his family in the Peloponnese during the period of the Ottoman threat (XXXIV. 7). Their relationship was typical of the socio-political relationships among the Byzantine bureaucrats.