Abstract This article focuses on two scientific expeditions sent by the Iberian crowns to the Americas between 1770 and 1790: Alexandre Rodrigues Ferreira´s nine-year journey to the Amazon region from 1782 to 1792 and Hipolito Ruiz´s botanical expedition to the Viceroyalty of Peru and Captaincy of Chile, 1777-1788. The aim is, firstly, to analyze how science was put at the service of the Empire’s interests. Those who led these expeditions were armed with new enlightened knowledges and new scienctific instruments to perceive American nature and natural resources. On the basis of travelers’ gaze and theirs scientific practices, this article highlights some differences between the two empires and territories they embodied.