Understanding Alexander von Humboldt’s role in his brother’s Americanist linguistics requires an appreciation of his American explorations. In today’s eastern Venezuela, Alexander engaged in linguistic-ethnographic fieldwork with the Chaima or Kumaná (Cariban); later, he met Quechua-speaking people in the Andes and Nahuas in Mexico among other Indigenous Americans, providing valuable sociolinguistic insights for Wilhelm. Alexander’s map of explorations closely reflects Wilhelm’s early inventory of American grammars, suggesting Alexander as a primary source of Wilhelm’s early American documents. Although sometimes chastised as a colonizer, Alexander proved unusually empathetic to Indigenous peoples, recognized sociohistorical continuities from pre-Columbian to post-contact societies, and addressed issues of colonial society from a hemispheric perspective, including exploitation and slavery. While visiting the United States on his return to Europe, Humboldt voiced his abolitionist position, but gained few insights about Native Americans’ fate as part of the Louisiana Purchase of 1803 or the developments leading to their removal.