A theoretical estimation is made of properties of the superdiamagnetic fluid and composite suspensions (colloidal solutions of particles of superconducting material dispersed in liquid solvent at T<T_c) that were previously proposed by the authors. It is shown that the superdiamagnetic fluid has an anomalously high initial diamagnetic susceptibility. It is further shown that such a fluid can be regarded as a model system for the investigation of phase transitions. The magnetisation curve of a composite suspension (in which a magnetic fluid serves as solvent) is investigated; the behaviour of this curve can be either para- or diamagnetic, depending on the applied magnetic field.