The existence of a second ion population in addition to (mostly) protons and electrons is a common signature of laboratory and space plasmas, and even a small admixture may modify the plasma properties significantly. A bi-ion fluid approach is used to describe the collisionless coupling between the two ion populations. It is shown that the appearance of additional wave modes leads to new types of stationary nonlinear structures, such as bi-ion solitons and a magnetic pile-up boundary (or ion composition boundary). We highlight effects observed at comets and Mars. ©2000 American Institute of Physics.