Abstract
This article examines the historical context and particular case of Italian merchant Guido Mondones (also named as ‘Modones’) who sold petroleum as medicine in Spain in the late sixteenth century. The first two-thirds of the article uses printed sources as a way to demonstrate that this merchant was likely not disadvantaged by being a foreigner and itinerant. Nor would he have been considered a suspect for selling oil with healing properties, as it was a fully accepted practice, including among university-trained professionals. All the materials for Mondones’s particular case are archival and contained within a lawsuit from Valladolid that contains a wealth of information about the merchant’s relationship with legal and medical authorities. Through these sources, we learn that he managed to be financially successful by navigating Spain’s particular medical–legal landscape at the time and skillfully defending himself against accusations of tax evasion and selling false medicine.