The experience of determining the ethnic contrast of the borders of the republics in the Ural-Volga region is presented based on a methodology that relies on assessing interethnic marriage rates between the titular nations of the republics and the Russian population. To achieve this, the study introduces an indicator called the Ethnic Contrast Coefficient, or the Soroko coefficient. The calculation of the coefficient is based on the assessment of «interethnic distances» defined by E. L. Soroko for pairs of major ethnic groups represented in the Ural-Volga region. The research revealed that the most ethnically contrasting borders in the region are those of Bashkortostan (excluding the section with Tatarstan), while the least contrasting are the borders of Udmurtia, Chuvashia, and Mordovia with neighboring Russian regions. The primary factor influencing the ethnic contrast of the borders between the republics of the Ural-Volga region is the religious factor, with the highest Soroko coefficient observed at the border between Chuvashia and Tatarstan.