The aim of this thesis was to determine how REE behave in reductive and oksidative conditions. Field research took samples of Jurassic and Paleogene bauxite from different locations in Istria. In this paper, samples from the profile of Paleogene bauksite, a total of six, were processed in detail. Laboratory research performed a detailed chemical analysis, analysis of iron compounds soluble in DCB and oxalate, and analysis of mineral composition using X-ray diffraction. Important conclusions derived from this paper are as follows: 1. pH / Eh conditions control the mineral composition of bauxite, the distribution of REE and major elements during bauxitization and early diagenesis. Jurassic and Paleogene red bauxites were formed in vadose conditions, gray bauxites were formed as a result of changes in the hydrological regime when the interaction of bauxite with saline porous waters in reductive conditions resulted in the formation of sulfide minerals, and white bauxites were formed by continuous destruction of plants in stagnant poor conditions which led to the removal of iron around the roots. 2. La / Y ratio values indicate that basic conditions generally prevailed in bauxite during bauxitization, but during bauxite formation and early diagenesis there were significant changes in pH / Eh conditions in the bauxite deposit that affected the mobilization and redistribution of REE. The lowest content of ΣREE in grey Jurassic bauxiteand the distribution of REE in white Jurassic bauxite indicate the existence of reductive conditions. The identified negative and positive cerium anomalies in Jurassic bauxite are a clear indicator of REE mobilization in acidic / reductive conditions. 3. A comparison of the results obtained by the DCB method and the oxalate method shows that well-crystallized iron oxides and oxyhydroxides predominate in the samples. 4. X-ray diffraction proved that all analyzed bauxites of Jurassic age contain boehmite, chlorite and anatase, and the dominant mineral in the samples of Paleogene bauxites is boehmite.