* Definition of the K or P-value
: The K or P-value always only refers to a single species or substance:
: with:
:* concentration of species i of a substance in the octanol-rich phase
:* concentration of species i of a substance in the water-rich phase
: If different species occur in the octanol-water system by dissociation or association, several P-values and one D-value exist for the system. If, on the other hand, the substance is only present in a single species, the P and D values are identical.
: P is usually expressed as a common logarithm, i.e. Log P (also Log P or, less frequently, Log pOW):
: Log P is positive for lipophilic and negative for hydrophilic substances or species.
* Definition of the D-value
: The P-value only correctly refers to the concentration ratio of a single substance distributed between the octanol and water phases. In the case of a substance that occurs as multiple species, it can therefore be calculated by summing the concentrations of all n species in the octanol phase and the concentrations of all n species in the aqueous phase:
: with:
:*  concentration of the substance in the octanol-rich phase
:*  concentration of the substance in the water-rich phase
: D values are also usually given in the form of the common logarithm as Log D:
: Like Log P, Log D is positive for lipophilic and negative for hydrophilic substances. While P values are largely independent of the pH value of the aqueous phase due to their restriction to only one species, D values are often strongly dependent on the pH value of the aqueous phase.