This study investigated the association between formal features, such as text volume, session length and duration, response latency, and waiting time, and the impact of counseling. The analysis was based on 603 text message counseling sessions at a child helpline and connected information about the formal features of the sessions and the effects on clients. The results showed that sessions characterized by more text volume from the counselor in each message, but with fewer messages from beginning to end, were more effective than sessions without these characteristics. Furthermore, session duration was associated with a positive impact, whereas counselor response latency was not. This indicates that clients might benefit from the asynchronous affordance of texting as long as the counselor responds promptly and with dense messages. We also found that the impact measured at end of session predicted the impact measured 2 weeks after the child or young person received counseling.