Cost-driven downstream process development and optimization is only possible when a complete process concept is either modeled or set up in laboratory scale, which requires comprehensive data or respective time and effort. Before that stage the decisions to apply or discard alternative purification steps are based on practical feasibility, experimental yield and purity improvement, without estimates for their cost-efficiency within the process. This article introduces two new parameters to overcome that problem. The purification performance index (PPI) weights the improvement in product purity achieved by a single step and relates it to the boundaries of the process. In this way the purification performance is assessed as a percentage of the required total purification. The separation cost indicator (SCI) combines purification rating and yield losses with cost data. Thereby cost-efficiency of single purification steps or step combinations can be estimated without the need of a complete process concept. (C) 2010 Elsevier B.V. All rights reserved.