ABSTRACT.A general axiomatic framework is developed for analyzing alternative measures of interactivity between the sectors of a given activity system (such as the industrial sectors of an economy). Within this overall framework, we analyze an ordinal class ofadditive‐ratio measureswhich has been applied to a wide variety of systems, including both economic and ecological systems of activities. Our main result is to show that such measures can be characterized by four interactivity axioms. In addition, a wide variety of alternative measures are compared to additive‐ratio measures in terms of this axiom sys