Similarly to many other sample extraction techniques, efficient extraction of very polar compounds with electromembrane extraction (EME) is difficult. To date, the best known strategy to improve the mass transfer of these compounds is the addition of an ionic carrier, often bis(2-ethylhexyl) phosphate (DEHP) to the supported liquid membrane (SLM). DEHP is known to work by providing ionic interactions with basic compounds, to improve the partitioning into the SLM. In this work, the behavior of DEHP during extractions was studied for the first time. Interestingly, substantial amounts of DEHP was found to leak from the SLM into the aqueous sample at pH > 4. Due to this leakage, the ion-pair formation between analytes and DEHP was moved from the sample/SLM interface (interfacial complexation) to the bulk of the sample solution (bulk-sample complexation), which improved the mass transfer of polar bases considerably. Based on this, an extraction procedure for eight polar bases with log P values from +0.7 to −5.9 was developed and optimized. The optimization demonstrated that extraction of more polar analytes was favored by bulk-sample complexation. With optimized conditions, extraction from biological samples such as urine, protein-precipitated plasma, and raw plasma were performed with recoveries >40%, except for a few analytes. In addition, the extraction system could be operated under robust conditions with relatively low current (<70 μA for plasma), and provided low variability (<16% RSD), as well as good clean-up efficiency. These findings are an important step in further scientific anchoring of EME, and development of the technique towards selective extraction of very polar substances from complex biological matrices.