In this paper part of the results obtained by a research project called "Primary Teachers' Thinking about the Concept of Volume and its Teaching", performed from 1997 to 2001, are reported. This paper focuses in one of the two main objectives of the aforementioned research: To describe the mental object volume of the participant teachers. The experience took place in a five-session workshop directed to in-service teachers and mathematics advisors of the Public Educational Bureau (Secretaria de Educacion Publica). The sessions were audio- and video-taped and a set of questionnaires was posed in the first session. The transcriptions of these recordings, as well as the answers obtained by the questionnaires, were analyzed in order to attain the planned objectives. [For complete proceedings, see ED500860.]