Phthalic acid diesters are additives in a variety of materials that can end up in landfills. ln a previous laboratory landfill simulation study, it was fouod that degradation intermediates such as phthalic acid monoesters and phthalic acid accumulate during the change from acidic to methanogenic conditions in municipal solid waste. In the present investigation, leachates from a series of fullscale youog landfill cells were analysed over time for dimethyl, diethyl, dibutyl, butylbenzyl, and di(2-ethylheliyl) phthalate (respectively designated DMP, DEP, DBP, BBP, and DEHP), and their corresponding monoesters monomethyl, monoethyl, monobutyl, monobenzyl, and mono(2 ethylhexyl) phthalate (MMP, MEP, MbutP, MbenzP, and MEHP, respectively), as well as ophthalic acid. One landfill cell was created in each of three consecutive years by deposition of the same type of waste in July and August. The pH, volatile fatty acids (VF As), and total orgaulc carbon (TOC) were measured to characterise development of the degradation phases in three landfill cells, which revealed early acidogenic to initial methanogenic stages. Analysis of the phthalate compounds showed that levels of the degradation products were below the detection limit in the acidogenic leachates but exceeded concentrations of their corresponding diesters in leachates from cells in the initial methanogenic phase. Maximmn and average concentrations of phthalic acid were 50 and 23 mg/L, respectively, and the corresponding values for the other phthalates were 430 and 27 µg/L. The concentrations of all phthalates decreased during the evolution of stable methanogenic conditions. The results were in aggrement with the landfill simulation reactor study.