A mold inhibitor is described which is a combination of a salt of propionic acid and an essential oil. The essential oil is selected from the group that includes but is not limited to cinnamon oil, eucalyptus oil, lemongrass oil, palmrosa oil, rosemary oil, sage oil, cassia oil, and thyme oil. The essential oil of cinnamon bark (Cinnamomum zeylanicum) has a synergistic effect with propionic acid in mold inhibition. The new mold inhibitors have improved efficacy, are less corrosive and are cost effective.