On the basis of microhardness (H) data measured at room temperature only for a number of polymers in the glassy state, a linear correlation between H and the glass transition temperature T-g, has been found (H = 1.97T(g) - 571). By means of this relationship, the deviation of the H values from the additivity law for some multicomponent and/or multiphase polymeric systems can be accounted for. The latter usually contains a liquidlike soft component and/or phase with T-g below room temperature. A completely different deformation mechanism in comparison to systems with T-g above room temperature is invoked. A novel expression for the hardness of polymers in terms of crystallinity of the single components and/or phases, the T-g values, and the mass fraction of each component is proposed. This expression permits the calculation of (i) the room-temperature H value of amorphous polymers, mainly containing single bonds in the main chain, provided T-g, is known, and of (ii) the contribution of the soft liquidlike components (phases) to the hardness of the entire multiphase system.