The rapidly growing field of phosphoproteomics has led to a strong demand for procedures enabling fast and reliable isolation and enrichment of phosphorylated proteins and peptides. During the past decade, several novel phosphopeptide enrichment methods based on the affinity of phosphoryl groups for titanium dioxide (TiO(2)) have been developed and tested. The ultimate goal of obtaining comprehensive phosphoproteomes has, however, been found difficult to achieve and the obtained results often vary, dependent on the enrichment method and protocol used. In the present study, the physical chemistry of the phosphopeptide binding to TiO(2) is investigated by means of measurements using a quartz crystal microbalance with dissipation monitoring (QCM-D). Special emphasis is put on the effect of the degree of phosphorylation of the phosphopeptide, the impact of the primary amino acid structure, and the role of electrostatic interactions. The results show that, in general, adsorption of phosphopeptides follows the Langmuir model and that the affinity for the TiO(2) surface increases in a nonlinear fashion with increasing degree of phosphorylation. An exception was detected, however, where positive cooperativity between the peptides existed and the Langmuir model no longer applied. The source behind the cooperativity could be traced back to the primary amino acid structure and, more specifically, the presence of positively charged amino acids in positions that enable electrostatic interaction with phosphoryl groups on neighboring peptides. Regardless of the net peptide charge, the TiO(2)-phosphopeptide interaction was for all phosphopeptides investigated found to be mainly of electrostatic origin. This study highlights and explains some of the most common problems with the TiO(2)-based enrichment methods used today.