The Motivational Interviewing Skill Code (MISC) is a coding system developed to measure adherence to motivational interviewing (MI). MI is an effective clinical style used in different treatment situations. Counsellors practising MI have to follow general principles and avoid certain traps. In the present study, the content of the MISC is compared with the general principles of MI and the traps to avoid in MI. Investigation of the content validity raises some questions. All general principles are represented but the traps to avoid in MI are not fully covered. The consequences of this under-representation are shown in transcripts of a selection of well-conducted MI training sessions. The reliability of the MISC was investigated by having five independent coders code 39 MI training sessions of different counsellors. The reliability of the MISC is reasonable. The five coders agreed to a large extent on the absolute ratings but the intraclass correlations were low. Although the MISC can be a useful research tool for process research of MI, it remains a labour-intensive instrument and for teaching and practice audit development of a more simple coding system is recommended.