The influence of sulfur dioxide on heat resistance was measured in 0.05 M, pH 3.3 tartaric acid solution and in various fruit juices. In tartaric acid solution, as little as 5 mg/L SO2 had a detectable effect; 100 mg/L reduced D80 from 123 min to 6.1 min, and at the 100 mg/ L, temperatures as low as 65°C caused a lethality. In fruit juices, 100 mg/L reduced heat resistance by 50% or more. Raising the pH from 3.3 to 5.0 neutralized the activity of sulfur dioxide indicating that it was the undissociated, molecular form that sensitized the ascospores to heat.