Gabapentin was used as a model pharmaceutical compound with susceptibility to polymorphic transformation as a function of environmental and mechanical stress. The utility of 13C CP/MAS NMR and XRPD as stability-indicating methods to quantify polymorphic transformation kinetics was investigated. Polymorphic Form II and III were distinguishable based on their chemical shift and distinct diffraction peak differences. Reproducible and accurate quantification of polymorphic composition in the presence of selected excipients was demonstrated using both signals from 13C CP/MAS NMR spectra and XRPD patterns. The effect of excipients on polymorphic transformations (Form II → III) was determined by measuring the transformation after co-milling. Both 13C CP/MAS NMR and XRPD were capable of measuring polymorphic composition in co-milled excipient mixtures without excipient peak interference. The amounts of Form III present in co-milled mixtures containing colloidal silicon dioxide, starch, hydroxy propyl cellulose and dibasic calcium phosphate were 8.7, 21, 33, and 39 mol%, respectively. A quenching procedure for obtaining 13C CP/MAS NMR spectra and environmentally-controlled XRPD were devised to determine polymorphic transformation kinetics of co-milled excipient mixtures during storage.