Co-fermentation is the process fermentation one or more grape varieties simultaneously. While this process is not done often, it is a common practice in the Northern Rhone, as well as Australia, where Syrah, a red grape, and Viognier, a white grape, are co-fermented. Generally, the amount of Syrah far exceeds the percentage of Viognier (where the Syrah makes up 80% or more of the mix). The process of co-fermentation supposedly increases the texture and color in the final product.

Field blends, which are made up of varieties that are grown in the vineyards together, are another place where co-fermentation is commonly seen.