Mass spectrometry (MS) is a uniquely informative technique in the characterization of copolymers, where spectra prominently feature peak clustering. The spacing of these clusters, in general, is dominated by the spacing of one repeat unit, and contained herein is the theory to explain this observation. Extension of this theory also explains the more subtle observation that, even though the spacing is generally that of one unit, occasionally, the spacing between the maxima of adjacent clusters shifts by that of the other unit. Furthermore, the theory predicts that, in the low molecular weight region of the spectrum, there is a total switch to the spacing of the other unit along with asymmetric peak clusters that have a “sawtooth” shape. The analysis uses the Gaussian, log–normal, and Schulz–Zimm models as well as the random coupling hypothesis to explicitly demonstrate that (1) the major peak cluster spacing naturally arises from the unit in the copolymer with the widest distribution, as measured by the sc...