Background: Art materials have a central role in art therapy. The way a client interacts with art materials – material interaction – is an important source of information in art therapy assessment in adult mental health. The aim of this study was to develop the categories of material interaction and examine the relationship between material interaction, the art product and clients’ psychological characteristics. Methods: Three sequential focus groups were conducted, in which 47 art therapists participated. Results: A large number of art therapists recognised that observing material interaction enabled them to gain a perspective on client’s psychological functioning. Styles of material interaction could be placed on a continuum from “affective” to “rational” and provided valuable information about a client’s affective and rational characteristics. Conclusions: Art therapists were able to describe clients’ material interaction and psychological characteristics based on the analysis of the formal elements of the clients’ art product. The results provided a preliminary theoretical base for art therapy assessment in adult mental health.