The highly dynamic and heterogeneous molecular ensembles characterizing intrinsically disordered proteins (IDP) in solution pose major challenges to the conventional methods for structural analysis. Electrospray ionization-mass spectrometry (ESI-MS) allows direct detection of distinct conformational components, effectively capturing also partially folded and short-lived states. We report the description of two complementary fragments (1-186 and 187-284) of the IDP Sic1, a cyclin-dependent protein kinase inhibitor of yeast Saccharomyces cerevisiae. Structural heterogeneity is noted in both cases, but the two fragments reveal slightly different conformational properties. The results are consistent with previously reported differences between the two protein moieties and corroborate the feasibility of IDP conformational analysis by ESI-MS.