AbstractThe ability of natural and synthetic hinokitiol, as well as a water soluble derivative (hinokitiol sodium salt), to protect wood against fungal attack was examined. Synthetic and natural hinokitiol provided similar protection. All three materials exhibited similar antifungal activity against Aspergillus niger and Penicillium citrinum on yellow poplar wafers at concentrations of 1 mg/mL or greater. Fungal attack by Gloeophyllum trabeum or Trametes versicolor was completely inhibited in soil block tests in wood treated with any of the three extracts at concentrations of 20 mg/mL or greater. The water soluble hinokitiol sodium salt was highly susceptible to leaching, and blocks subjected to leaching had little resistance to fungal attack. The results suggest that further formulation development will be necessary to produce a water-soluble hinokitiol system that can resist leaching and retain biological activity.