Vickers microhardness has been measured at room temperature on a numerous compounds and alloys containing Ce (4f) and/or U (5f) metals ( all together on 43 samples with different compositions represented by the parameter x) belonging to the following series of systems: Y1-xUxRu2Si2, Ce1-xYxRu2Si2, Ce1-xYxCu2.05Si2, Ce1-xLaxCu2.05Si2, Ce1-xUxAl2, Ce1-xUxNiSn. The significant changes were observed for all examined series of the systems with the discernible maximum or minimum in the recorded hardness curves. Whereas the maximum could be easily explained by the available theories, explanation of the minimum needs additional studies. (C) 2004 Kluwer Academic Publishers.