Individual and combined attachment dimensions within the dyad may affect therapists' experiences in ways that hinder mentalizing and threaten the alliance. Previous research on a particular therapists' mentalizing ability named the elaboration of countertransference experience (ECE) suggests that attachment may affect therapists' mental processes, which in turn are involved in alliance variations between dyads and over time. In this study, these findings are extended by examining therapist, client, and combined attachment dimensions as moderators of the association between ECE and working alliance. Forty-four assessments from the first 10 psychotherapy sessions of 12 independent dyads were examined. The ECE dimension of reflection/conjecture was positively associated with the alliance bond and total score in therapeutic dyads that differed more in attachment. Possible in-session dynamics at stake are discussed.