Effective utilization of acid whey remains a challenge for the Greek yogurt industry. The most common uses of acid whey are (1) application to agricultural fields, (2) incorporation into animal feed, or (3) as a feed source for anaerobic digesters. Acid whey contains several components with the potential for value-added consumer products. Wellness tonics or drinkable vinegars, which are high in acetic acid, are becoming popular among consumers for their health benefits. One of the main components in acid whey, lactose, can be converted to acetic acid through microbial fermentation. Vinegar has a 2-step fermentation process: anaerobic fermentation by yeast to convert sugar to ethanol, and aerobic fermentation by Acetobacter to convert ethanol to acetic acid. Brettanomyces clausennii, now popular in craft beers, is unique in its genus for its ability to use lactose. Under anaerobic conditions, this yeast can convert lactose to ethanol, and under aerobic conditions to acetic acid. Fermentation by B. clausennii has the potential to simplify vinegar production, allowing production of novel, dairy-based acetic acid beverages. The objective of this study is to initially characterize aerobic fermentation of lactose in acid whey to acetic acid by B. clausennii. Acid whey (250 mL) was inoculated with B. clausennii to obtain a concentration of ~10⁵ cfu/mL. The fermentations were incubated at 30°C with shaking. Samples were taken every 24 h for 3 d. Levels of acetic acid were measured with an enzymatic assay kit. The experiment was repeated 3 times and the samples were analyzed in duplicate. A Student’s t-test was used to determine significant differences in acetic acid levels in the treatment between d 0 and d 3. After 3 d the acetic acid concentration in the fermentation significantly (P < 0.05) increased from 0.09 ± 0.01 g/L to 2.70 ± 0.27 g/L. The control acetic acid levels were 0.09 ± 0.00 g/L on d 0 and 0.10 ± 0.01 g/L on d 3. These results show that there is potential to create a one-step fermentation process of acid whey to vinegar with B. clausennii. Further studies will include optimization of fermentation conditions and sensory evaluation.