The quantitative description of diffusive and convective mass transport of small solutes in peritoneal dialysis is dependent on accurate determination and appropriate expression of the concentration of investigated substances. For small solutes which easily equilibrate between dialysate and plasma the solute concentration in plasma should be expressed per volume of plasma water (aqueous concentration) and not per volume of whole plasma. Furthermore, the Donnan effect should be taken into account for electrolytes if measured by flame photometry. The common practice of expressing solute concentration per volume of whole plasma (plasma concentration) may result in substantial errors in calculated values of peritoneal transport parameters. To quantify these errors we compared plasma versus aqueous dialysate to plasma ratios (D/P), diffusive mass transport coefficients (KBD), and sieving coefficients (S) for 28 6-h single-dwell studies using glucose 3.86% dialysis fluid. For all substances except glucose non-corrected plasma D/P overestimated corrected aqueous D/P at 360 min by 2% (potassium and sodium) to 8% (creatinine) and, as assessed by the Pyle-Popovich model, non-corrected KBD overestimated true KBD by 12% (potassium) to 41% (urea). Similar results were also obtained for KBD estimation using the Garred model and KBD estimated during dialysate isovolaemia. The use of aqueous instead of plasma concentrations resulted in a substantial change of S for urea but not for the other investigated solutes. These results emphasise the importance of expressing small solute concentrations per volume of plasma water and not per volume of whole plasma in calculations of D/P ratios and mass transport coefficients.