Biorefineries constitute representative examples of multifunctional systems which are able to produce, similarly to conventional petroleum refineries, a wide range of chemicals (pharmaceutical constituents, plastics, food additives etc.), energy carriers and power through the optimal use of diverse biomass forms (wheat straw, oils, wood chips, municipal solid waste). For this purpose, biorefineries typically comprise a complicated, integrated network of physical and chemical transformation processes, such as mechanical and physical biomass pretreatment, pyrolysis, gasification, catalytic and enzymatic reactions, and downstream purification processes. For the environmental sustainability assessment of these complicated production systems, Life Cycle Analysis (LCA, ISO-Norm 14040) is considered as a widely acceptable methodology from scientists and engineers including, however, the debated aspect of partitioning the impacts among the co-products' in the biorefinery product portfolio. The aim of this study is to present the influence of the various allocation approaches on the LCA results of biorefinery products. The framework of this analysis systematically incorporates the steps of the LCA methodology as described in the ISO norms and estimates the impacts related with the products of interest, taking into account the contribution of the co- and by-products in the overall production path. For this reason, two wider approaches were adopted, the attributional which describes the impact of the production process itself from a retrospective point of view, and the consequential which focuses on the changes in the level of the output (as well as consumption and disposal) of a product, including market effects from increasing or decreasing demand for the study product, having therefore a more prospective point of view. Several scenarios which describe the possible options for handling those products, were developed and assessed based on different allocation methodologies, namely system expansion (substitution method) and partitioning methods according to the mass, thermal and economic values of the co-products. The estimation of the life cycle impacts of the processes was performed using the Global Warming Potential (GWP), Cumulative Energy Demand (CED) and RECIPE methodologies which provide an assessment of the burdens through the associated LCA indicators. The outcome of this approach provides a range of LCA metrics emphasizing at the variation of the results according to the followed allocation methods and to identify those properties of products (physical, economic, thermal) and system factors (processes to be substituted from the renewable ones, degree of utilization of co- and by-products from the markets etc.) which dominate the LCA results.