Chemical stability of UF adhesives is an important hygienic property. This paper deals with chemical stability of UF adhesives modified with glutaraldehyde (GA) and collagen. GA was added to the adhesive to increase the structural stability of the adhesive. Collagen was added in the form of acid hydrolysate; the hydrolysate is chrome tanned leather waste. Formaldehyde content in plywood was measured through FESYP method; and formaldehyde emission from cured UF adhesive film was measured through “chamber method” according to the STN 49 0030 and STN EN 717-1 standards. The results showed that simultaneous presence of collagen and GA in the UF adhesive mixture reduced the amount of formaldehyde in plywood, and reduced the formaldehyde emission from cured UF adhesive film, as well.