Abstract This article analyzes an unpublished corpus of 131 letters written in Spanish by four Dutch merchants during the seventeenth-century (1669–1677). The aim of the investigation is to determine whether the variation found in these letters was internal, external, or both, and to study the factors that favored each source. To do so, I will look at vowel variation in this corpus. Since standard vowel variants were being selected in seventeenth-century Spanish, the research questions addressed in this work are the following: Did multilingual merchants acquire the emerging standard of the moment? Did multilingual merchants contribute to the selection of standard variants? Did multilingual merchants acquire the sociolinguistic variation of that period? How did multilingual merchants differ from L1 speakers and why? How did linguistic and social factors influence this variation? The results revealed a great amount of intra- and inter-speaker vowel variation, and this variability was a matter of both internal and external factors. Therefore, this study argues that, while multilingual merchants acquired some of the standard variants that were being selected in the emerging standard Spanish of the seventeenth century, they reported linguistic variation that can be explained considering the context of use and the socio-economic conditions underlying their acquisition of linguistic repertoires.