Organotin compounds are well established as fungicides in organic solvent‐based wood preservative formulations for out‐of‐ground contact applications. In this context investigations have continued throughout the year on the biocidal effectiveness of alternative candidate tributyltin de‐rivatives towards two wood‐destroying fungi Conio‐phora puteana and Co dolus versicolor. In addition, a mini‐block soil contact burial test has been initiated, in conjunction with the Building Research Establishment, Princes Risborough Laboratory, designed to assess the activity of three organotin compounds against soft rot soil fungi. This type of test is useful for screening candidate chemicals for use in the preservation of timber in ground‐contact situations. Initial results with tributyltin compounds, including water‐soluble tributyltin ethanesulphonate, are much better than expected. Biological and analytical data have been obtained for accelerated ageing procedures (leaching by water or elevated temperature) on T.B.T.O.‐treated Scots pine sapwood. It has been demonstrated that heating (to 60°C) influences the breakdown of the tributyltin preservative to lower alkylated products. Results of a study, in collaboration with the Swedish Wood Preservation Institute, Stockholm, on timber taken from an out‐of‐ground contact trial indicate that there is a strong correlation between weight losses of samples exposed to Coniophora puteana and the amount of T.B.T.O. present in the wood. Experiments are in hand to establish the reason why the total tin content appears to decrease with time as degradation proceeds. Chemical analyses were also carried out on blocks of Pinus sylvestris treated with a limited number of other tributyltins, Bu3SnX, to assess the effect of the X group on degradation.