AbstractElectromembrane extraction is affected by acid–base equilibria of the extracted substances as well as coupled equilibria associated with the partitioning of neutral substances to the supported liquid membrane. A theoretical model for this was developed and verified experimentally in the current work using pure 2‐nitrophenyl octyl ether as supported liquid membrane. From this model, extraction efficiency as a function of pH can be predicted. Substances with log P < 0–2 are generally extracted with low efficiency. Substances with log P > 2 are generally extracted with high efficiency when acceptor pH < pKaH − log P. Twelve basic drug substances (2.07 < log P < 6.57 and 6.03 < pKaH < 10.47) were extracted under different pH conditions with 2‐nitrophenyl octyl ether as supported liquid membrane and fitted to the model. Seven of the drug substances behaved according to the model, while those with log P close to 2.0 deviated from prediction. The deviation was most probably caused by deprotonation and ion pairing within the supporting liquid membrane. Measured partition coefficients (log P) between 2‐nitrophenyl octyl ether and water, were similar to traditional log P values between n‐octanol and water. Thus, the latter have potential for pKaH − log P predictions.