In 1898 two indigenous families stayed for few monthes in Buenos Aires; in that situation Robert Lehmann-Nitsche documented a vocabulary of Selk'nam language, that was verified and enlarged in 1902 during a fieldtrip in Fire Land. The source follows the basic schema of the linguistic questionnaire published by Georg von der Gabelentz few years earlier; the German anthropologist also included a few sentences and short texts in Selk'nam as well as ethnographic and metalinguistic remarks written in German. The article provides access to this source, which so far had only been partially published by Lehmann-Nitsche. We describe the context of the survey, we analyse issues related to the vocabulary content and we state the mechanisms used by Lehmann- Nitsche for the recontextualization of the data previously gathered in the edition he made in 1913. His documentation task is also connected with those developed by other agents who researched on selk'nam in the period between the XIX and XX centuries.