The Organisation for the Prohibition of Chemical Weapons (OPCW) has long been reflecting on how to prevent the ‘re-emergence of chemical weapons’ as the verification of the destruction of declared stockpiles continues to approach completion. To deal with this shift in emphasis, a functional rebalancing of activities and resources will likely be required as the OPCW seeks to ensure that the implementation of the Chemical Weapons Convention remains relevant and effective in future operating environments. Despite much expert attention, studies that explore and characterise the nature of change within the OPCW are scarce, resulting in gaps in our knowledge regarding the different actors involved, and how processes of change unfold. This stems from ontological and theoretic positions embedded within some mainstream approaches used to examine international organisations, which often treat secretariats as bureaucratic ‘black boxes’, characterising change as the product of state machinations. Moreover, change is often treated as episodic and exceptional, arising from deliberate and controlled efforts. Drawing on archival research at the Sussex-Harvard Information Bank and participant observation within the OPCW, this thesis investigates how our understanding of changing within the OPCW can be enhanced if we treat the Secretariat as a purposive actor involved in these processes. This enhanced understanding enables a deeper exploration of what change looks like and how it unfolds, providing insights that can support contemporary development. The thesis uses culture theories to present a unique assessment of the Secretariat through the interaction between formal/official and informal/unofficial cultural manifestations. This gives the Secretariat character, opening the black box and allowing its role in changing to be considered. Then, inspired by the work of Andrew Pettigrew, an analytical framework based on a process metaphysics ontology is employed to examine processes of changing. Three longitudinal case studies are used to examine these processes in response to perceived challenges posed by chemical terrorism and non-state actors. This reveals how States Parties and the Secretariat co-create through long-run processes of change and continuity. Evidence for taking seriously the role of bureaucratic bodies in organisational development is presented, exploring how agency can be variously conceived of, and how changing often tends to be multiplicitious and not confined to a single category. The thesis has theoretical implications. Approaches that arbitrarily ignore particular actors within (international) organisations should be treated with caution, and where possible inclusivity should be sought. Mainstream theories about change tend to be similarly exclusive, prioritising or prescribing a particular form or type of change. This thesis has demonstrated that a variety of forms of changing can co-exist, suggesting that expanding organisational change approaches might be fruitful. An important insight to emerge is that bureaucracies tend toward dysfunction rather than the ideal-type, and using cultural approaches can open up new spaces for examining relationships between structure and agency across different levels, and bring new dimensions to our understanding of secretariats. Practical implications include demonstrating how the Secretariat has contributed to organisational capacity and capability to respond to perceived challenges around chemical terrorism and non-state actors. In doing so, it provides new perspectives on how the OPCW develops. The research argues that as the OPCW functionally rebalances, attention to organisational geographies and identities will need to be part of human resource strategies, as the cultural analysis reveals areas of tension. Finally, the Secretariat are co-creators of organisational changing, and although their inputs and impacts can be hidden, indirect, or informal, this research reveals they are productive. This suggests that more evidence-based research is needed to examine the role of secretarial components in organisational changing. During times of normative stress and functional uncertainty, this must be a priority.