Coal seams of Late Eocene (Tanjung Formation) and middle Miocene age (Warukin Formation) are present in the Barito Basin of Borneo. The seams are laterally continuous and can be traced over at least 20 km. The thickness of three Eocene seams in the TAJ Pit–1D mine ranges from 1.4 to 3.4 m. These seams (from base to top: D, C, B) were studied to reconstruct their peat–forming environment and to compare the characteristics of Eocene and Miocene coals. The comparison reveals differences in peat types, flora and climate. The study is based on 38 Eocene coal and six non–coal samples, each representing a stratigraphic interval of 0.2 m. The samples were analyzed for ash yield, carbon and sulphur contents, and maceral composition. Organic geochemical parameters (incl. biomarkers) were obtained on every second coal sample to obtain information on the peat-forming vegetation and its diagenetic alteration. The Eocene seams were deposited in rheotrophic mires with palm/fern–dominated vegetation. Transitions to local ombrotrophic mires cannot be excluded. Very low to low sulphur contents (max. 0.9 wt%) speak against a marine influence, despite a general coastal plain setting. Upward increasing sulphur contents in the lower seam D and the upper seam B are interpreted to reflect transgression, which ended peat accumulation. The seam C formed in a rheotrohpic seam with a relatively high water level. This favored the accumulation of aquatic plants. Miocene coals from the Warukin Formation in the Barito Basin were studied recently in a companion paper. The comparison of Eocene and Miocene coals shows major differences, which include: Miocene coals are significantly thicker (up to 50 m) and display a cyclic structure. Miocene coals accumulated in ombrotrophic mires dominated by more decay resistant angiosperm trees (and subordinate gymnosperms). Therefore, preserved plant tissues are more abundant in Miocene coals. In addition, Miocene coals contain a significantly higher amount of root–derived macerals (suberinite). Leaf–derived macerals (cutinite, fluorinite) and resinite are ubiquitous in Miocene coals, but even slightly more abundant in Eocene coals. At least part of the resinite in Miocene coals, but not in Eocene coals, is derived from dammar resin produced by Dipterocarpaceae. Fungal activity, recorded by high funginite percentages, was high in low-ash ombrotrophic and high-ash rheotrophic mires both in Eocene and Miocene times.