Previous investigations on Saterland Frisian report a large vowel inventory, including up to 20 monophthongs and 16 diphthongs in stressed position. Conducting a cross-dialectal acoustic study on Saterland Frisian vowels in Ramsloh, Scharrel, and Strücklingen, the second objective is to identify merged vowel categories as well as acoustic dimensions, which may enhance the discrimination of spectrally adjacent categories within the crowded vowel space of Saterland Frisian. All vowels were elicited in a /hVt/ frame. Acoustic measurements included vowel duration, mid-vowel F1 and F2, and the trajectory length and spectral rate of change. The results confirm large inventories for the three varieties of Saterland Frisian but also show that some categories have undergone a merger. The cross-dialectal comparison revealed an effect for Scharrel, in which the close-mid monophthongs are produced with higher F1 values than in Strücklingen and Ramsloh and the diphthongal trajectories are shorter than in the other two varieties. The findings contribute to the description of an endangered minority language and add to the question which acoustic cues are used in languages with large vowel inventories to maintain or enhance the existing contrasts.