Phase diagrams of a set of three monodisperse polystyrene lattices in the presence of non-adsorbing polymer molecules (hydroxyethyl cellulose) have been determined. Polymer molecules induce reversible phase transitions involving gas, liquid and solid-like states. We find that the phase diagrams can have two distinct topologies: fluid-solid only or fluid-solid plus fluid-fluid equilibria. The preference for one topology is governed by the ratio 6 / ~, where 6 and U are the polymer and colloidal particle sizes. Moreover, a three-phase coexistence is noticed instead of a triple point. The attractive effect of polymer molecules is investigated through light scattering experiments and we propose direct vizualisation of the critical fluctuations in these colloidal systems.

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