Objective: We explored moderators of cognitive behavioural therapy (CBT) treatment effects and predictors of outcome at 12-month follow-up in the CODES Trial (N = 368) comparing CBT plus standardised medical care (SMC) vs SMC-alone for dissociative seizures (DS). Methods: We undertook moderator analyses of baseline characteristics to determine who had benefited from being offered CBT 12 months post-randomisation. Outcomes included: monthly DS frequency, psychosocial functioning (Work and Social Adjustment Scale - WSAS), and health-related quality of life (Mental Component Summary (MCS) and Physical Component Summary (PCS) SF-12v2 scores). When moderating effects were absent, we tested whether baseline variables predicted change irrespective of treatment allocation. Results: Moderator analyses revealed greater benefits (p < 0.05) of CBT on DS frequency for participants with more (≥22) symptoms (Modified PHQ-15) or ≥ 1 current (M.I.N.I.-confirmed) comorbid psychiatric diagnosis at baseline. The effect of CBT on PCS scores was moderated by gender; women did better than men in the CBT + SMC group. Predictors of improved outcome included: not receiving disability benefits, lower anxiety and/or depression scores (PCS, MCS, WSAS); shorter duration, younger age at DS onset, employment, fewer symptoms and higher educational qualification (PCS, WSAS); stronger belief in the diagnosis and in CBT as a “logical” treatment (MCS). Some variables that clinically might be expected to moderate/predict outcome (e.g., maladaptive personality traits, confidence in treatment) were not shown to be relevant. Conclusion: Patient complexity interacted with treatment. CBT was more likely to reduce DS frequency in those with greater comorbidity. Other patient characteristics predicted outcome regardless of the received intervention.