This paper on the Wisconsin variety of Walloon (a Romance language originating in southern Belgium) discusses the shift from contrastive length to contrastive tenseness among high vowels, and the subsequent reduction of the Wisconsin Walloon phonemic inventory by way of a merger. Belgian Walloon exhibits a four-way contrast among high front vowels: long and short, and rounded and unrounded. Heritage speakers of Wisconsin Walloon predictably contrast in tenseness instead of length, and do not contrast between rounded and unrounded short (now lax) vowels. Data from native speakers of this moribund and severely understudied heritage variety of Walloon are used to support this hypothesis. Implications for the phonological status of tenseness and sound change processes in heritage languages are briefly considered.