The synthesis and characterization of sequenced copolymers is a relatively underdeveloped field despite the fact that biological polymer properties are known to depend directly on sequence. The concept of sequence fidelity is introduced and defined for repeating sequence copolymers as the ratio of repeat units of the target sequence to the total number of repeat units. NMR spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF MS) are used to characterize the sequence fidelity of repeating sequenced copolymers comprising poly(lactic-co-glycolic acid)s. Both characterization methods are found to have both advantages and weaknesses with regard to identifying and quantifying error in these copolymers. A method for using routine MALDI ToF MS data to quantify sequence fidelity is proposed and applied to a series of copolymers with synthetically defined error rates. The MS-determined sequence fidelities are comparable to those obtained by NMR analysis.