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 región 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.Resumo Este artigo ocupa-se de duas expedições científicas enviadas a América pelas coroas ibéricas no final do século XVIII: as viagens de Alexandre Rodrigues Ferreira às Capitanias de Grão-Pará, Rio Negro, Mato Grosso e Cuiabá, e Hipólito Ruiz e Antonio Pavón ao Vice-reino do Peru. O objectivo será analisar de que forma a botânica serviu os interesses imperiais e a percepção que os naturalistas que protagonizaram estas expedições, armados com novos saberes ilustrados e novos instrumentos, tiveram da natureza americana e dos recursos naturais que esta oferecia. A partir da perceção destes viajantes e de suas práticas científicas, destacam-se algumas diferenças entre os dois impérios e os territórios que os integravam.  Resumen Este artículo se ocupa de dos expediciones científicas enviadas a América por las coronas ibéricas a finales del siglo XVIII: los viajes de Alexandre Rodrigues Ferreira a las Capitanías de Grão-Pará, Rio Negro, Mato Grosso y Cuiabá (1783-1792) e Hipólito Ruiz y Antonio Pavón al Virreinato peruano (1777-1788). El objetivo será analizar la forma en que la botánica se puso al servicio de los intereses imperiales y la percepción que los naturalistas que protagonizaron estas expediciones, pertrechados con los nuevos saberes ilustrados y los nuevos instrumentos, tuvieron de la naturaleza americana y de los recursos naturales que ofrecía. La mirada de estos viajeros, y sus prácticas científicas, ponen de relieve algunas diferencias entre ambos imperios y los territorios que los integraban. 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 región 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.