The copolymerization of a-methylstyrene (MSt) and styrene (St) to form random and blocky copolymers is well-known. Strongly alternating copolymers of this type are rarely described, mainly because of difficulties linked to their characterization. Indeed, the chemical structures of St and MSt are sufficiently similar that quantitative determination of the sequence distribution is rather difficult. Here, we report a complementary pair of analytical methods based on nuclear magnetic resonance (NMR) and high-resolution mass spectrometry, respectively, to convincingly assess the structure of such copolymers. First, sequence-controlled poly(a-methylstyrene-alt-styrene) copolymers were obtained based on the single-unit addition principle, enabling unit-by-unit addition of the two monomers. The alternating character of these copolymers was then detected via carbon-13 (C-13) NMR (C-13 NMR) and selective heteronuclear multiple bond correlation 2-dimensional NMR (SHMBC NMR) on aromatic quaternary carbons. Diad probabilities were quantified and used to calculate statistically derived quantities, reflecting the nature of the copolymer sequence distribution. In addition, HRMS and tandem mass spectrometry were performed to further confirm the alternating tendency of these copolymers, and they revealed a very narrow chemical composition distribution and fragmentation patterns specific to a primarily alternating structure.