Many researchers suggest that communication is most effective if a high level of interactivity between participants is involved. As yet, however, there is neither consensus on how interactivity is defined, nor on how it can be measured. The present study is aimed at finding an operational definition of interactivity that can be used in almost all communication contexts. Additionally, based primarily on three recent reviews on the concept, the study proposes an objective instrument for determining different levels of interactivity. The instrument consists of a checklist for observing the presence of a set of representative characteristics such as synchronicity, timing flexibility, control over content, physical presence of participants and the extent to which participants in a communication situation use their senses. The applicability of the instrument is demonstrated by determining the interactivity levels of two exemplary communication situations. © The Author(s), 2009.