Petroleum products may contain up to thousands of individual hydrocarbon compounds, which vary widely in environmental behavior and ecotoxicity. Environmental risk assessment of these complex substances is facilitated by use of the hydrocarbon block method (HBM). The HBM assigns similarly behaving hydrocarbons to the same blocks and then relates release rates of petroleum products to environmental concentrations and human intake rates by means of so-called environmental fate factors and human intake fractions of the hydrocarbon blocks. We have derived such fate factors and intake fractions and associated uncertainties with a library of representative hydrocarbon structures. Fate factors and intake fractions of over 1500 individual representative hydrocarbons have been modeled according to the EU Technical Guidance Documents. Fate factors and intake fractions for the chosen hydrocarbon blocks are then estimated from average values obtained for the individual compounds in the blocks. Fate factors and intake fractions of the hydrocarbon blocks vary by up to 10 orders of magnitude for the different emission scenarios and compartments/spatial scales. The highest fate factors are obtained for the exposure scenarios with the highest emission intensity (the local emission scenarios) and for the most direct exposure chain. Uncertainties introduced by the “blocking” method are typically smaller than 30% and rarely larger than a factor of 3. Various hydrocarbon blocking schemes of different resolution were tested, and all schemes investigated appeared to perform satisfactorily. The use of the library-based HBM was illustrated by assessing the aquatic ecological risk of gas oil, emitted to water. The aquatic risk characterization ratio of gas oil hydrocarbons in regional freshwater is estimated to be 6 × 10(−5) per unit of emission (1 kg/d released 1:10:100 to local, regional, and continental scales, respectively), with an uncertainty of 32%. In view of other uncertainties in environmental risk assessment, the precision achieved with the procedure presented here is judged to be reasonable.