Periodic wave trains can be induced at the surface of liquid layers when the liquid is heated from above. These wave trains are an oscillatory regime of the Marangoni-instability. They are studied in a circular annular container, in which a wave synchronization is possible. Two counter-rotating (periodic) wave trains can be observed at a moderate level of supercritical Marangoni-number values. During the head-on collisions of the waves a negative phase shift occurs. This is evidence of the nonlinear interaction of the waves and their "solitonic" properties.