Nonpolar regions in biological molecules are investigated as a determining factor governing their electrospray ionization (ESI) mass spectrometric response. Response is compared for a series of peptides whose C-terminal residue is varied among amino acids with increasingly nonpolar side chains. Increased ESI response is observed for peptides with more extensive nonpolar regions. The basis for this increase is examined by comparing values of nonpolar surface area and Gibbs free energy of transfer for the different amino acid residues. Comparisons of response with octadecylamine are also made, and this highly surface-active ion is observed to outcompete all other analytes in ESI response. These observations are rationalized on the basis of the equilibrium partitioning model, which is used successfully to fit experimental data throughout the concentration range for several two-analyte systems. This model suggests that because excess charge exists on ESI droplet surfaces, an analyte's relative affinity for the droplet surface determines its relative ESI response. Increased nonpolar character, which leads to enhanced affinity for the surface phase, results in more successful competition for excess charge and higher ESI response.