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 scaled standard deviation, (2) the spacing shift naturally occurs due to the marginal probabilities away from the spectrum maximum, and (3) the low molecular weight switch is a natural consequence of the tail of the distribution of the unit with the widest distribution. Results are provided to predict which unit in the copolymer will govern the major peak cluster spacing, how often the spacing will shift to that of another unit in the middle and high molecular weight regions of the spectrum, the molecular weight and composition of the maximum peak in every cluster, and the molecular weight below which the spacing will be that of the another unit. We believe that our results are the first to provide tangible theory to explain the previously unknown origins of these empirically observed phenomena. © 2013 American Chemical Society.