The antidecay resistance of beech-wood samples (120 x 8.5 x 8.5 (MM)) treated with tributyltin fungicides gradually decreased due to prolongation of their natural ageing: tributyltin fungicides (TBTO, TBTS, TBTCA, TBT-DEDTK were applied by pressure impregnation technique in ethanole solutions (c = 0. 1%, 0.33% or 1%); treated beech-wood samples were naturally aged without their contact with ground (from 0 to 4 months); the antidecay resistance of treated and aged beech samples was tested against the brown-rot fungus Serpula lacrymans and white-rot fungus Coriolus versicolor, and then evaluated on the basis of their weight losses (DmF) and their relative impact bending strength decreases (DAF(rel)). The statistical evaluations indicated that: toxic values [kg/m3] of tributyltin fungicides have been significantly increased due to natural ageing processes; tributyltin fungicides were more effective against the brown-rot fungus Serpula lacrymans; both criteria DmF and DAF(rel) have been, on the whole, comparable when assessing the early stages of rot in the unsatisfactorily treated beech samples, although decreases of DAF(rel) were several-times higher than of DmF; however, for the higher stages of decay (DmF > 8%) just the AmF criterion could more sensitively express the further increasing differences in rot.

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