The environmental variability during the Mazovian/Holsteinian interglacial is better understood thanks to the results of multidisciplinary palaeobotanical studies. The perfectly preserved and abundant material from Nowiny Żukowskie in SE Poland has been the subject of numerous palaeobotanical analyses. The results both of initial pollen analysis and of the examination of plant macroremains provide a detailed view of changes in the palaeoenvironment of this area during the Mazovian/Holsteinian interglacial. Originally, the water basin was mostly the habitat of plants indicative of low trophy. The frequent occurence of swamp plants evidences a change in hydrological and climatic conditions consistent with the intra-interglacial climatic oscillation. In the subsequent part of the optimum, an expansion of swamps with Aracites interglacialis and Dulichium arundinaceum was recorded. The development of a peat bog overgrown by i.a. Sphagnum sp., Eriophorum vaginatum, and Andromeda polifolia was also observed. The growth of swamp and peat vegetation resulted in the nearly complete disappearance of aquatic vegetation, apart from species typical of the climatic optimum of the Mazovian/Holsteinian interglacial: Brasenia borysthenica and Aldrovanda dokturovskyi. The close of the interglacial was marked by the intensive development of peat bog and swamp communities with Carex rostrata, Menyanthes trifoliata, and A. interglacialis. The intensive increase in the number of A. interglacialis during the period described as the “birch oscillation” supports the hypothesis of noticeable changes in hydrological conditions at that time. The end of the described period is typified by a deterioration of climatic conditions, indicated by the increase in values for Betula humilis, B. nana, and Juniperus communis.