Abstract What explains how and why international humanitarian organizations (IHOs) sometimes succeed, and sometimes fall short, in their efforts to negotiate unimpeded access during armed conflicts to implement humanitarian relief programs? This article presents a plausibility probe of a theory to answer this question. According to this explanation, humanitarian access obstruction results from the interaction between one factor that is structural (the level of governments' security dependence on members of the Organization for Economic Co-operation and Development's Development Assistance Committee) and a second factor that is agentic (the scale of IHOs’ negotiation mobilization). This article probes the plausibility of this theory using a mixed-methods research design. Quantitative analysis entails analysis of an original dataset created on humanitarian access obstruction during armed conflicts between 2005 and 2021. The dataset includes 186 unique country–year observations spanning 32 countries. Qualitative analysis entails case studies—based primarily on 89 semi-structured interviews that the author conducted—on humanitarian access negotiations between traditional IHOs and governmental actors in the context of the Donbas conflict in Ukraine and the Syrian Civil War.