This paper is devoted to the surname changes performed through administrative channels in the interwar period. The research is based on the announcements of the “Official Gazette of the Republic of Poland” in 1929. The author describes main reasons for the decisions of surname changes taking into account characteristics of avoided surnames and chosen demographic tendencies, especially those connected with the age and profession of applicants. People of Jewish origin, Poles and representatives of other nationalities showed different motives for surname changes. Jews most frequently changed their surnames due to legal reasons — they wanted to legalize the unlawful use of a surname of the so-called ritual father. The changes carried out under the motive of assimilation occurred definitely less often. Non-Jewish applicants changed mainly appellative names, especially those derived from words related to animals. After comparing tendencies occurring before and after World War II one concludes that besides legal and assimilation factors which are particular to the pre-war decades (connected with the ethnic, legal and religious situation of the time), the remaining reasons for the surname changes are universal and do not distinguish the pre-war period from that of the post-war.