Sudden gains are large and stable decreases in clinical symptoms between consecutive therapy sessions. This work examined the frequency and possible determinants of sudden gains in Cognitive Therapy for Social Anxiety Disorder, comparing face-to-face (CT) and internet-based (iCT) formats of treatment delivery. Data from 99 participants from a randomised controlled trial were analysed. The frequency of sudden gains was high: 64% and 51% of participants experienced a sudden gain in CT and iCT respectively. Having a sudden gain was associated with lower social anxiety symptoms at posttreatment and follow-up. There was evidence of reductions in negative social cognitions and self-focused attention immediately prior to the sudden gain, contrasting with no prior reductions in depression symptoms. Ratings of session videotapes in CT showed that clients' statements indicated greater generalised learning in sessions immediately prior to gains, compared to control sessions. This may suggest a role for generalised learning in facilitating these large symptom reductions. There were no significant differences in results between the CT and iCT treatment formats, suggesting that the therapy content appears to play a more important role in determining participants' large symptom improvements than the medium of treatment delivery.