Roofing membranes produced with ketone-ethylene-ester (KEE) and polyvinyl chloride (PVC) are defined within the standard specification for KEE-based sheet roofing, ASTM D6754, Standard Specification for Ketone Ethylene Ester Based Sheet Roofing, in which it is specified that such membranes must contain at least 50% (wt.) KEE. Until the recent release of ASTM D8154, Standard Test Methods for 1H-NMR Determination of Ketone-Ethylene-Ester and Polyvinyl Chloride Contents in KEE-PVC Roofing Fabrics, in which the determination of KEE content is based on nuclear magnetic resonance (NMR) spectroscopy, no method was sufficiently precise or accurate to measure the KEE content within 1% or better. In this paper, a description is provided of the development and the basic process behind the use of 1H-NMR to establish KEE content in KEE/PVC membranes as described in ASTM D8154. To develop a quantitative method to assess KEE concentration in roofing membranes, several NMR approaches were used, including both solid-state and liquid-state NMR. Approaches using solid-state NMR, including carbon-13 and proton spectroscopy, proved inadequate for the quantification of KEE because of insufficient spectral resolution. Liquid-state NMR proved to be a better approach, but accurate results are achieved only after the KEE-PVC blend is extracted from the roofing membrane through a multistep process. The liquid-state NMR work led to two methods to quantitatively measure the KEE content in blends with PVC. Method A is the simplest, and its accuracy is better than 2.3%. Method B is more protracted, but its accuracy is better than 0.6%.