For correct formulation of the problems on the optimal design of composite materials, one needs to know how the requirements for different composite properties can combine with one another. Investigation of optimal combinations of material properties is carried out within the preliminary design of composites. In the process, one draws the surfaces of composite ultimate properties, which show the relations between ultimate attainable values of several material characteristics, simultaneously meeting the requirements for each material characteristic. Such relations describe a material with a variable interior structure in the same way as does a set of engineering constants for a material with a fixed structure. The present paper covers ultimate properties of unidirectional fibrous composites. For simplicity our consideration is restricted to a plane problem. To draw the surfaces of ultimate properties for hybrid composites we use the adaptable priority technique in the vector optimization. Compromise domains (Pareto-optimal solutions) are obtained for two-component composite materials on the basis of the parametric analysis. The examples of ultimate properties are given for typical composites in the cases when the requirements for different composite properties were simultaneously imposed, i.e. for rigid, thermoelastic, thermal, and dissipative characteristics. Rational applications of different composite types are discussed.