Formaldehyde is present everywhere in our environment. Because of its harmful properties to the health we want to lower its level to a minimum. Free formaldehyde is present in particle boards, because of polycondensation resins: MF, UF, MUF, etc. In the course of time free formaldehyde releases from particle boards, most often used indoors. Influence of wood bark particles on emission of free formaldehyde was investigated. 10 particle boards from beech wood (Fagus sylvatica L.) with different amounts (20, 35, 50, 65 and 80 %) of spruce bark (Picea abies L.) were tested. 10 samples (25x25x16 mm) were made from each particle board; then by standard glass method SIST EN 717-3 formaldehyde was measured. It was found out that increasing the bark share the amount of free formaldehyde was strongly reduced, but just to some level. Polyphenol substances like extractives, lignin and tannins, representing the main part of the bark, are the reason for this to happen.