A new carbon-13 NMR method is presented for the quantitative determination of monomer sequence distributions and average sequence lengths in propylene-1-butene copolymers. Complete carbon-13 NMR chemical shift assignment was finalized: the peaks corresponding to comonomeric sequence distributions are assigned in terms of previous assignments, and other peaks are formulated after observation of two reference systems: a propylene homopolymer and a propylene copolymer with 8.17 mol % 1-butene. Particular attention was focused on determination of the P-centered triad sequence distributions, and the monomer sequence distributions were quantitatively determined, based directly on either the isolated carbon-13 NMR peaks or on the sequence distribution relationship equations. In addition, the effect of the propylene sequence configuration on the quantitative determination of the monomer sequence distributions of PP-centered tetrads was minimized. Molecular microstructural information was extracted with the highest possible accuracy. This method is reliable for analysis of the monomer sequence distributions in propylene-1-butene random copolymer.