The experimental and theoretical concept concerning the magnetic properties of 2-dimensional arrangements of small particles with different strengths of interaction is discussed. Taking into account recent results on isolated small single-domain particles, a representation for a whole scale of possible structures from these to closely packed ferromagnetic films is given, using a molecular field formalism. The interaction considered may be much weaker than the full ferromagnetic exchange coupling. The resultant collective magnetizationMof such aggregates of small particles may be considerably lower than their spontaneous magnetization Ms. A transition temperature TAtowards occurrence of superparamag-netism may appear that is well below the Curie temperature Θ of the individual particles. These effects seem to be consistent with experimental data obtained by a number of authors using various methods of measurement. Observations of time effects at magnetization reversals in very thin films also support the aforementioned model.