Objectives. Contemporary relational theories consider clients' ability to move between multiple self-states and clients' ability to experience and process emotions to be two therapeutic processes inherently connected and fundamental to growth and change in psychotherapy. The current research aimed to empirically explore these theoretical assumptions by (1) examining the between-and within-client temporal association between clients' levels of ability to move between multiple self-states and clients' emotional experiencing and processing over the course of therapy, and by (2) testing whether this association would be more prominent for clients presenting good treatment outcomes. Method. Based on treatment outcome measures, two contrasting groups were selected, nine successful and nine unsuccessful. Ninety session transcripts (five per client) were analysed using the Two-Person APES (TPA), an extension of the Assimilation of Problematic Experiences Scale (APES), for the measurement of the ability to move between self-states. Sessions were also analysed using the Experiencing Scale (EXP), for the measurement of emotional experiencing. For all predictors, we disaggregated withinand between-client effects. Results. No association was found between EXP and TPA levels. However, at the between-client level, an interaction effect was found indicating a positive correlation between TPA and EXP only for the good-outcome cases, whereas a negative correlation between these variables was found for the poor-outcome cases. Conclusions. The ability to move between multiple self-states and accessibility to emotional experiencing may be seen as complementary processes enhancing the effectiveness of therapy. However, a high level of one process combined with a low level of the other may be a sign of poor therapeutic outcome. Practitioner points Clients' capacity to negotiate between conflicted self-states and their ability to experience and process emotions are two determinants working in a synergistic way to predict improvement in clients' symptoms. The presence of only one of these psychological processes without the other might be an indication of poor therapeutic outcome.