Infra-red (i.r.) end-group analysis is used to determine the hydroxyl and carboxyl end-group concentrations and therefore the number-average molecular weight of poly(ethylene naphthalene-2,6-dicarboxylate)(PEN) film samples in the solid state. The end-group i.r. spectrum of a sample is obtained by subtracting the i.r. spectrum of a deuterated control sample from the experimental sample i.r. spectrum. We have used integral absorbances obtained from fitted absorption bands in the i.r. spectra of PEN for the calculation of end-group concentrations. Using this method, the limitations of chemical methods of determination of end-group concentrations caused by the poor solubility of PEN are avoided. In addition, corrections can readily be made to allow for difference in crystallinity between the control and the experimental samples. The extinction coefficients for the OH stretching vibration in the hydroxyl and for the carboxyl end-groups have been determined for PEN as well as for poly(ethylene terephthalate). Both the hydroxyl and the carboxyl end-group concentrations, and the number-average molecular weight of the samples, can then be easily determined from the i.r. absorbances.