To understand techno-science as an activity that is changing the world, it is necessary to analyze the value systems that guide techno-scientific actions. This article proposes a model of analysis, distinguishing ten components of such actions and ten types of values involved in their evaluation processes. Contrary to the theory of rational decision making based on maximizing one value or utility function, the present analysis is based on the notion of the gradual satisfaction of a plurality of values. A distinction between nuclear and orbital values is also suggested. In the case of orbital (or peripheral) values, processes of techno-scientific choice can be represented by means of an evaluation matrix whose numerical values change over time. However, nuclear values must first have been satisfied to some extent. Thus the role of nuclear values should be represented by means of successive axiological sieves. © 2003 Elsevier Science Ltd. All rights reserved.