SummaryLangmuir adsorption isotherms are unlikely to be appropriate when applied to the adsorption of charged phosphate ions on charged oxide surfaces. It is more likely that the adsorption model proposed by Bowdenet al.(1973, 1974 and 1977) for adsorption on variable charge variable potential surfaces will be applicable. Theoretical isotherms were calculated from this model for a series of pH values and ionic strengths. It is shown that these isotherms can be split into two or more Langmuir isotherms in much the same way as has been done by various authors for experimental isotherms. The parameters of the Langmuir isotherms behave in the same way as those derived from the experimental isotherms. The charging characteristics of the theoretical isotherms are also similar to that observed experimentally.Because the model which is based on a single type of adsorption site is closely similar to the experimental isotherms, it is erroneous to split the isotherms into a series of Langmuir isotherms unless there are gooda priorireasons.