Systematic downstream process development and optimization requires extensive experimental evaluation of process alternatives. Two key performance indicators, the purification performance index and the separation cost indicator, can be utilized to enable estimation of cost-efficiency in early stages of process design. However, being convenient for rating single purification steps they cannot estimate the performance of combinations of these steps. For this reason purification fingerprints are introduced in this paper which characterize purification steps or mass separating agents due to their contaminant-specific selectivity in a descriptive and comprehensive way. They uncover strengths and weaknesses of purification steps in removing certain contaminants. Furthermore, the fingerprints of steps once characterized are valid throughout the process under the assumption of unchanging driving force ratios for all components. Thereby, efficient step combinations can be derived from the combination of contrary fingerprints. In this way the complete purification progress of a downstream process can be tracked and optimally designed from the start. (C) 2010 Elsevier B.V. All rights reserved.