There is growing evidence that coaching is effective. However little is known about the process variables critical for coaching success. This article investigates the role of the two interpersonal behaviour dimensions affiliation and dominance in the formation of a positive coaching relationship and in coaching success. The interaction of 33 coach client dyads was videotaped during their first coaching session. The verbal and non-verbal affiliation and dominance behaviour of both actors was analysed with the Discussing Coding System. Findings show that a coach’s dominance behaviour in the first session predicts positive ratings by the client concerning goal attainment at the end of the five-session-coaching process. Similarity of coach and client in terms of dominance and affiliation predicts positive ratings by the client on relationship quality and goal attainment. Practical implications and limitations are discussed.