We have used potentiometric titrations to measure the pK values of the ionizable groups of proteins in alanine pentapeptides with appropriately blocked termini. These pentapeptides provide an improved model for the pK values of the ionizable groups in proteins. Our pK values determined in 0.1 M KCl at 25°C are: 3.67±0.03 (α-carboxyl), 3.67±0.04 (Asp), 4.25±0.05 (Glu), 6.54±0.04 (His), 8.00±0.03 (α-amino), 8.55±0.03 (Cys), 9.84±0.11 (Tyr), and 10.40±0.08 (Lys). The pK values of some groups differ from the Nozaki and Tanford (N&T) pK values often used in the literature: Asp (3.67 this work vs. 4.0 N&T); His (6.54 this work vs. 6.3 N&T); α-amino (8.00 this work vs. 7.5 N&T); Cys (8.55 this work vs. 9.5 N&T); and Tyr (9.84 this work vs. 9.6 N&T). Our pK values will be useful to those who study pK perturbations in folded and unfolded proteins, and to those who use theory to gain a better understanding of the factors that determine the pK values of the ionizable groups of proteins.