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In the ®rst part of this paper we brie¯y describe experimental (octanolywater partitioning) as well as computational approaches to quantifying molecular lipophilicity. The central section focuses on the hydrophobic fragmental constant approach (Sf -system) as developed by Rekker and his group, starting in the early seventies. The original approach has been extended and revised a number of times; the most recent updating is presented here. It is followed by a detailed description of how to apply the correction factor C M . The practical procedure of Sf-calculations is described for some examples and the validity of these calculations is veri®ed by comparison with other calculation methods and experimental data.

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