Coexistence of two modes of ion-acoustic waves in two-ion-species plasmas is investigated both theoretically and experimentally. Two ions are combinations of H2, He, Ne, Ar and Xe. The boundary of existence of two modes in the plane of an electron–ion temperature ratio and heavy–light ion mass ratio is obtained from the two-species ion-acoustic dispersion relation with a real frequency and a complex wavenumber. The boundary is given approximately by Te/Ti = 2Mh/Ml, where Te and Ti are temperatures of electrons and ions, respectively, and Mh and Ml are masses of heavy and light ions, respectively. The experiment is perfomed with a double-plasma device. Ion-acoustic waves are transmitted by applying a sinusoidal pulse to the anode of the source plasma. A single mode is observed for Ne–Ar, He–Ne, Ne–Xe and Ar–Xe plasmas and two modes are detected for H2–Ar, H2–Ne and He–Ar plasmas.