A multicenter study was conducted to determine the relationship between intraperitoneal volume and solute (e.g., urea, creatinine) transport as determined by the dialysate to plasma (D/P) ratio and mass transfer area coefficient (MTAC). Two 4-h peritoneal equilibration tests were conducted on each of 12 pediatric peritoneal dialysis patients (mean age, 10.8 yr; range, 0.2 to 19.2 yr). One test exchange volume was 900 mL/m2 body surface area (BSA), and the other was 1,100 mL/m2 BSA. Dialysate samples were collected at 0, 30, 60, 120, 180, and 240 min. Blood samples were drawn at 0 and 240 min. Solute equilibration was significantly more rapid with the 900 mL/m2 BSA exchange volume than with the 1,100 mL/m2 exchange volume when evaluated by the D/P ratio. In contrast, no differences in solute transport were noted with either exchange volume when assessed with the MTAC. In conclusion, solute D/P ratios vary with changes in the intraperitoneal volume, necessitating the use of a standardized exchange volume for the reliable interpretation of evaluations such as the peritoneal equilibration tests. In contrast, the use of the MTAC allows for an accurate assessment of solute transport over a wide range of exchange volumes.