It is well recognized that the pK'value for the salt of a weak acid or base is significantly influenced by the ionic strength of the solution. In the case of phosphate, the theoretical value for pK2 in an infinitely dilute solution is 7.181 at 370 C.(1) and in a solution of ionic strength com-parable to that of plasma is approximately 6.8. The small variations in ionic strength ordinarily encountered in plasma would not be expected to produce significant deviations from this latter value. However, in view of the wider range of ionic strengths which occurs in urine, values differing appreciably from 6.8 might be anticipated. In the present study an attempt has been made to evaluate the relationship between pK2'phos-phate and ionic strength in urine, and to define the usual range of values which might be encoun-tered in the course of acute phosphate loading experiments. A method has been devised for the determination of phosphate pK2'in urine and the experimental values have been compared with those for phosphate solutions of similar ionic strength. Thedata suggest that under the con-ditions of the present experiments the behavior of phosphate in urine conforms closely to that predicted for simple phosphate solutions by the Debye-Hfickel equation. It has further been demonstrated that the pK2'of phosphate in urine may vary significantly and that under some cir-cumstances failure to consider this variation may introduce appreciable errors into either the measurement or calculation of urinary" titratable acid."