This research was performed as part of the EU project FACT (contract SMT4-CT96-2135) on field tests out of ground contact and ageing methods. The results reported here are part of the work on the experimental lap-joint standard (ENV 12037). TNO investigated the TBTO content of 32 lap-joints by analysing the tin content of 90 samples. The lap-joints were treated with two different TBTO-concentrations (0.5%; 1.0%) and different treatment methods (brushing, dipping, vac-vac treatment). Analysis took place after initial treatment and 6 months after field testing above ground at differ-ent test sites in Europe. The samples for analysis were taken from the top layer (first 3 mm) and from the inner part of the lap-joint. Preliminary tests with different extraction methods (AWPA, CTBA and DPI-FS) for tin analysis in wood showed the highest recovery rates for the AWPA method. Therefore extraction of the wood samples was carried out in accordance to the AWPA-A6-89-4 method. The extracts were analysed by ICP-AES. Results of the tin analysis are discussed referring to field test site . treatment method and sample origin.