Indoxyl sulfate and p-cresyl sulfate are protein-bound marker molecules in chronic kidney disease. Recent findings suggest that indoxyl sulfate and p-cresyl sulfate directly contribute to the uremic syndrome. A method for quantification of p-cresyl sulfate and indoxyl sulfate total serum concentrations was developed. We used sodium octanoate as competitor to replace non-covalent binding of p-cresyl sulfate and indoxyl sulfate to albumin. Total, within-run, between-run and between-day imprecision for indoxyl sulfate and p-cresyl sulfate were all below 6%. The limit of quantification was 3.2microM for both analytes. Recovery, tested in hemodialysis patients, was 102% for indoxyl sulfate and 105% for p-cresyl sulfate. Deming regression demonstrated good agreement for indoxyl sulfate between this new method and an external HPLC method. Method comparison for p-cresyl sulfate of the new method with our in-house GC-MS method demonstrated good agreement, whereas method comparison with an external HPLC method revealed a small proportional bias. Sodium octanoate binding competition is a novel sample preparation that allows for direct quantification of indoxyl sulfate and p-cresyl sulfate.