RING-between-RING (RBR) ubiquitin ligases work with multiple E2 enzymes and function through an E3-ubiquitin thioester intermediate. The RBR module comprises three domains, RING1, IBR and RING2 that collaborate to transfer ubiquitin from the E2~Ub conjugate, recognised by RING1, onto a catalytic cysteine in RING2 and finally onto the substrate in a multi-step reaction. Recent studies have shown that RING1 domains bind E2~Ub conjugates in an open conformation to supress ubiquitin transfer onto lysine residues and promote formation of the E3 thioester intermediate. However, how the nature of the E2 influences the ubiquitin transfer process is currently unclear. We report here a detailed characterization of the RBR/E2-conjugate recognition step that indicates that this mechanism depends on the nature of the E2 enzyme and differs between UbcH5 and UbcH7. In the case of UbcH5~Ub an interaction with ubiquitin is necessary to stabilize the transfer complex while recognition of UbcH7~Ub is driven primarily by E2-RING1 contacts. Furthermore our analysis suggests that RBRs, in isolation and in complex with ubiquitin-loaded E2s, are dynamic species and that their intrinsic flexibility might be a key aspect of their catalytic mechanism.