An on-line SPE-CE system is described for the determination of insulin derivatives in urine, serum and plasma. By combining techniques based on different separation mechanisms, in this case reversed-phase SPE and CE, a more selective sample clean-up is obtained. The described on-line SPE-CE procedure is able to desalt and clean biological samples, resulting in more repeatable electrophoretic results as well as a good linearity for urine, serum and plasma samples spiked with insulin derivatives, thus proving the elimination of detrimental effects caused by the sample matrix. The on-line SPE-CE system was linear for urine, serum and plasma samples spiked with insulin derivatives between 5 and 80 mg/l. The repeatability in migration time was below 1% relative standard deviation (R.S.D.). The repeatability of the peak was better (<2.4% R.S.D.) when no off-line precipitation reaction (<6.2% R.S.D.) was used, proving the beneficial characteristics of on-line sample pretreatment procedures over off-line sample pretreatment procedures which are prone to sample losses and contamination.