Biphasic partition coefficients are important parameters in describing environmental behaviour of chemicals. Collander Equations are linear expressions linking the logarithms of solvent/water partition coefficients (Ksw). Despite their importance, the theoretical basis for these equations is not well developed, limiting their predictive capability. Using Regular Solution Theory, a general equation is developed relating log Ksw to the logarithm of the octan‐1‐ol/water partition coefficient (Kow). As an example of the applicability of the expression, calculated hexadecane/water (Khw) and heptane/water (Khepw) partition coefficients are in satisfactory agreement with available experimentally determined values for homologous solute series of alkanes, alkenes and alkylbenzenes. Agreement is less satisfactory for alkan‐1‐ols and chlorobenzenes. The theoretical treatment predicts that Collander Equations relating log Ksw to log Kow are overall curvilinear, but linear over limited log Kow ranges.