The KRAS peptide library is a part of our larger effort to understand peptide variants in shotgun proteomics. Herein, the interest is in discrimination of homologous peptides---a problem related to detection of peptide variants. The dataset contains MS/MS spectra of 400 synthetic peptides of form LVVVGA-XX-VGK. Each peptide is measured in separate LC-MS/MS run, thus the actual peptide for each dataset is known. In summary, the dataset contains annotated spectra of homologous peptides and can be used for development and optimization of peptide detection methods.