Objective To compare the contribution of “trial-sibling” and “unrelated” qualitative studies in complex intervention reviews. Study Design and Setting Researchers are using qualitative “trial-sibling” studies undertaken alongside trials to provide explanations to understand complex interventions. In the absence of qualitative “trial-sibling” studies, it is not known if qualitative studies “unrelated” to trials are helpful. Trials, “trial-sibling” and “unrelated” qualitative studies looking at three health system interventions were identified. We looked for similarities and differences between the two types of qualitative studies, such as: participants, intervention delivery, context, study quality and reporting, and contribution to understanding trial results. Results Reporting was generally poor in both qualitative study types. We detected no substantial differences in participant characteristics. Interventions in qualitative “trial-sibling” studies were delivered using standardised protocols, whereas interventions in “unrelated” qualitative studies were delivered in routine care. Qualitative “trial-sibling” studies alone provided insufficient data to develop meaningful transferrable explanations beyond the trial context, and their limited focus on immediate implementation did not address all phenomena of interest. Together, “trial-sibling” and “unrelated” qualitative studies provided larger, richer datasets across contexts to better understand the phenomena of interest. Conclusions Findings support inclusion of “trial-sibling” and “unrelated” qualitative studies to explore complexity in complex intervention reviews.