Lagoons can be used for the treatment of many kinds of waste-waters. This study is focused on the biological treatment of leachate from oil-shale ash heaps in lagoons. To be able to perform these studies in the laboratory in a reasonable time the lagooning process was simulated by a batch process. The influence of a number of parameters, such as temperature, nutrients, aeration and inoculation, on the treatment process was investigated. It was shown that the success of the treatment process, measured as removal of organic compounds, was dependent on nutrients and inoculum. It was fully possible to get a successful treatment also at a low temperature, even if the degradation was initially somewhat slower. The best treatment results were achieved with intermittent aeration (1 day aerobic, 6 days anaerobic). In this case the phenolic compounds (phenol, dimethylphenols and cresols) were degraded to below their detection limits (1 mg/l) and the overall degradation of organic compounds was 75 %.