A number of experimental studies have shown electric-field manipulation of nanowires dispersed in liquids. We demonstrate from first-principles that the electrical response of semiconductor nanowires in liquids must be described by considering, at least, two mechanisms for interfacial polarization: the classical Maxwell-Wagner interfacial polarization and the formation of an electrical double layer. We compare the theoretical predictions with our experimental data and with data published elsewhere and show that both mechanisms play an important role.