Alliance may impact psychotherapy outcomes both as a precondition that enables therapeutic work and an evolving process that is therapeutic in itself. This study examined the participation of the elaboration of countertransference experience (ECE) in alliance variation between therapist-client dyads early in therapy and within dyads over time. A total of 44 session assessments nested within 12 dyads were modeled through longitudinal multilevel analyses and utilized to examine the associations between the ECE dimensions of Immersion and Reflection and alliance components across 4 time points within the first 10 sessions of psychotherapy. Results supported the importance of initial ECE to explain differences in alliance between dyads, the particular relevance of ECE with clients presenting lower levels of personality organization, and the effect of personality difficulties on alliance change. Unexpected results were found concerning the correlations between ECE and alliance and their covariation over time. In conclusion, ECE dimensions appear to be involved in alliance formation, both in initial differences between dyads and in changes over time within the same case. ECE seems particularly important with more personality-disturbed clients. Future research should disentangle therapist and client contributions and examine the participation of ECE in the resolution of alliance ruptures. (PsycInfo Database Record (c) 2020 APA, all rights reserved).