The purpose of this study was to assess the bioprotectant performance of chemical modification wi th three different isocyanates (n-butyl, hexyl and l,6-diisocyanatohexane, BuNCO, HeNCO and HOI respectively) in Corsican pine (Pinus nigra Schneid) sapwood. Wood-isocyanate bond formation was verified by the increase in sample weight, volume and by infrared spectroscopy. Basidiomycete (Coniophora puteana, Gloeophyllum trabeum, Coriolus versicolor, Pycnoporus sanguineus) decay tests demonstrated protection by chemical modification. The relationships of fungal species, weight percent gain (WPG), decay induced weight loss and moisture content were examined. One of the brown rot fungi, ~. puteana, showed higher threshold protection values than the other fungi tested and the di isocyanate showed better perf ormance. Scanning Electron Microscopy and enzyme detection tests (cellulase and phenol oxidases) have been carried out in an attempt to gain a better understanding of the treatment performance. Chemical characteristics of the sound and brown rotted wood (~. puteana) have been examined using sulphuric acid, sodium chlori te and high performance liquid chromatography-HPLC (gel permeation chromatography-GPC) procedures to clarify the principles which govern isocyanate modifications and restrict fungal decay. A number of parameters were examined including lignin and holocellulose contents, holocellulose molecular weight and degree of polymerisation (OP and dispersity). These demonstrated that chemical modification changed the configuration of the original wood polymers. Al though preferential modification occurred at lower weight percent gains in the lignin fraction appreciable wood protection against C. puteana only occurred when the holocellulose fraction showed substantial changes due to chemical modification. To examine further the effect of moisture and loading of substituent groups within the outer layers of wood after chemical modification (BuNCO & HOI), tensile strength resistance to surface colonisation by soft rot fungi was undertaken utilizing thin wood strips after unsterile soil tests. Less modification was necessary to achieve protection against soft rot in this test.