Through its extensive use as a fuel oxygenate, Me tert-Bu ether (MTBE) is found nearly ubiquitously throughout the environment. To better understand the environmental fate of MTBE, fugacity models are commonly used. However, models developed by the scientific community and by governmental bodies differ in their predictions of relative MTBE concns. for relevant environmental compartments and of seasonal concn. variations; further, to date they have not considered the formation of transformation products. In this study, the sensitivity of predicted environmental concns. of MTBE and its two major degrdn. products, tert-Bu formate (TBF) and tert-Bu alc. (TBA), to all types of model input parameters is analyzed in a probabilistic sensitivity anal. This anal. allowed for an assessment of the most influential parameters for predicting soil, water, and air concns. and thereby provided insight into why previous modeling studies on MTBE differed. Further, the information from the sensitivity anal. was used to parametrize a multispecies transformation model for predicting European concn. levels of MTBE and, for the first time, TBF and TBA. Water and air concns. of MTBE predicted with the transformation model were in good agreement with measurements of environmental samples. No studies are available on environmental TBF and TBA levels to compare with model predictions; however, the modeling results indicate that, in the water phase, TBA concns. may reach appreciable levels. One major uncertainty identified regarding the prediction of TBA levels was the fraction of TBA formed from atm. MTBE and TBF.