The aim of this article was to investigate the impact of ageing heat treatment of 2101 lean duplex steel on changes in the microstructure and mechanical properties, mainly impact toughness. Steel was subjected to ageing treatments in the temperature range of 500-900°C with exposure times of 1, 10, and 100 hours. Light microscope examinations, hardness measurements and impact toughness tests were performed. The obtained results confirm that high-temperature service of duplex stainless steels should be avoided. Precipitation of secondary phases (mainly α’, gamma, gamma2 phases) strongly deteriorates mechanical properties of steels but some amounts of these phases may be acceptable in the microstructure depending upon the application of the steel. The test results obtained for 2101 lean duplex steel were compared with the results obtained earlier for higher alloy 2205 and 2507 duplex steel grades. The research showed that the loss of plasticity after ageing at 500°C affects the 2101 lean duplex steel the most, while ageing at higher temperatures, above 700°C, limits the plasticity of 2205 and 2507 steel grades to a much greater extent.