We perform numerical simulations of a simple model of one-patch colloidal particles to investigate: (i) the behavior of the gas–liquid phase diagram on moving from a spherical attractive potential to a Janus potential and (ii) the collective structure of a system of Janus particles. We show that, for the case where one of the two hemispheres is attractive and one is repulsive, the system organizes into a dispersion of orientationally ordered micelles and vesicles and, at low temperature (T), the system can be approximated as a fluid of such clusters, interacting essentially via excluded volume. The stability of this cluster phase generates a very peculiar shape of the gas and liquid coexisting densities, with a gas coexistence density that increases on cooling, approaching the liquid coexistence density at very low T.