This study focuses on the mid-centralvowel /a/-also known as the "sixth vowel"-in Mapudungun, the largest Amerindian language spoken in Chile. Although most studies agree that Mapudungun has six phonemic vowels and that the sixth vowel has two allophones, [w] and [a], there is no consensus regarding which variables are better at explaining the sixth vowel's variability, how to represent the underlying unit, or even whether the existence of both [w] and [a] are justified. To investigate this, several exploratory and inferential statistical approaches were used to analyse normalised F1, F2 and F3 values from 791 tokens of the sixth vowel, in order to determine: (a) whether there is evidence of [w] and [a] in the data; (b) if the linguistic variables stress, position of vowel in a word, and phonetic context have an effect on the acoustic values of the sixth vowel, and (c) the degree of overlap of this vowel with other phonological categories. The results showed that, contrary to expectations, there is not sufficient evidence to justify the existence of [(sic)], which suggests that the underlying unit is /(sic)/, that only the variability of F2 can be explained, in part, as due to the effect of linguistic variables, and that /(sic)/ overlaps mainly with the vowel space of /e/. These results are discussed in the light of internal and external factors of linguistic sound change, after which we suggest that this variety of Mapudungun might be experiencing the initial stages of merger by approximation, most likely due to external factors related to the interaction with a dominant language, in this case Spanish.