Abstract A life cycle assessment (LCA) approach was used to identify the processes and stages in organic waste composting that have the largest environmental impacts. The LCA included impacts associated with the collection of feedstock, production and distribution of compost, and its use as a replacement for peat for soil conditioning. The use phase of the compost product has not been included in previous LCA studies in the United States. Nine LCA impact categories were analyzed (global warming potential, ozone depletion, smog, acidification, eutrophication, carcinogens, non-carcinogens, respiratory effects, and ecotoxicity) using TRACI 2 methodology. The functional unit was defined as the collection, processing, transportation, and application of one tonne of compost that meets USEPA composting standards. The compost was produced at the Organics Material Processing and Education Center (OMPEC) at The Pennsylvania State University. The data used in the assessment was collected from seven composting windrows over thirteen consecutive months (December 2010–January 2012). Given the wide range of decomposition emission factors reported in the literature for methane (CH 4 ), nitrous oxide (N 2 O) and ammonia (NH 3 ), three emission scenarios were calculated: average, minimum, and maximum emission scenarios. Carbon dioxide (CO 2 ) emissions from the compost were considered biogenic and not included in the assessment. For all scenarios, compost processing was the stage with the largest environmental impact, with decomposition emissions contributing the most to global warming potential, acidification and eutrophication impact categories under the average and maximum emissions scenarios. To account for the avoided environmental impacts of peat mining and transport, these values were subtracted from the composting life cycle. The avoided impacts from peat replacement were higher than the impacts from composting for all categories, illustrating that using compost instead of peat results in net environmental gains. This study highlights the importance of minimizing life cycle impacts associated with CH 4 , N 2 O and NH 3 emissions during the decomposition process and the need for more consensus in the literature on emission values from composting processing.