This paper addresses the resistance to biological degradation of acetylated wood. Firstly, it presents results from an EN 252 test after 18 years’ exposure. The test is still running in Nordic NWPC test fields, Simlångsdalen in Sweden and Viikki in Finland. Results from this test were presented after 5 years’ exposure ( IRG/WP 97-30139), and showed that the resistance of acetylated wood with an acetyl content of about 20% was in the same range as that of wood treated to the high retention of the reference CCA preservatives. At acetylation levels above 20%, none of the samples was rated higher than rating 1, either in Simlångsdalen or in Viikki. After 12 years’ exposure the average index of decay was 51 in Simlångsdalen and 45 in Viikki for the medium level of acetylation (average acetyl content: 19,8%), compared to the high retention level of the NWPC Standard CCA preservative, where the index of decay was 41 and 31, respectively. Making the same comparison after 18 years’ exposure in Simlångsdalen, the index of decay was 56 for the high retention level of the NWPC Standard preservative. For the acetylated test stakes, the corresponding figures were 65 for the medium level and 60 for the high acetylation level (average acetyl content 22,0%). This implicates that in order to be comparable with the CCA preservative for 18 years, an acetyl content of above 22% is needed. This is true when exposed in a brown rot soil. However, it should be pointed out that in the test field in Finland the stakes with the high level of acetylation performed extremely well, confirmed by an index of decay of 12,5 compared to the high retention of the NWPC reference CCA preservative where an index of decay of 60 was obtained. Secondly, acetylated wood (acetyl content 22%) has also been evaluated after 10 years’ marine exposure at Kristineberg on the Swedish west coast. The test is still running and is conducted according to EN 275 (marine borer resistance). The test was started in 1999 and the X-ray rating now shows a moderate attack by marine borers, whereas the untreated controls failed after only one year. If the acetylation (acetyl content 21%) is followed by a post treatment with melamine resin (weight gain 19%) no attack could be observed after ten years.

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