This report describes the results of an interlaboratory comparison study held by TNO/MT and RIVM-LOC on the congener specific determination of the toxic congeners of PCDD's and PCDF's in fly ash of emission measurements of municipal waste incinerators using Strohlein sampler filters. The comparison included 12 samples of stack gas emissions of municipal waste incinerators at Zaandam (n=6), Alkmaar (n=3) and Leeuwarden (n=3), The Netherlands. Results of analysis of dioxins in fly ash samples of both laboratories were in good agreement when expressed on the amount of toxic equivalences (TEQ-values). The standard deviation of fully independent parallel analysis was better than +- 15%. Differences in amounts of the individual toxic congeners found by both laboratories were in general random in character, and were on average less than a factor of two for most of the congeners.<br>