Grammatical relations are central to the scientific study of languages, and yet the indigenous languages spoken in Amazonia often challenge conventional approaches to the notion of grammatical subject and object. This project, which will include on-site linguistic fieldwork, proposes a novel investigation into the the properties of grammatical relations in a sample of Amazonian languages spoken in the highly diverse Guaporé-Mamoré area in Brazilian and Bolivian Amazonia. More specifically, I will investigate the morphological and syntactic properties of subjects and objects in Differential Argument Marking configurations: differential subject marking, also known as non-canonical subjects, and differential object marking. This areal and cross-linguistic research will also encompass two grammatical domains that are directly connected to grammatical relations, namely subject properties and configurationality. The research in this project will deepen our knowledge of the grammatical landscape in the highly diverse and endangered indigenous languages spoken in the Guaporé-Mamoré region, and will also throw light on the linguistic convergence across the language families in the area---which can provide us with linguistic evidence for the history and patterns of contact among the Guaporé-Mamoré indigenous communities before the onset of European colonization.