We demonstrate the feasibility of using FT-IR spectra of aerosols and a multivariate calibration to estimate organic carbon (OC) from thermal-optical reflectance analysis. Using 800 IMPROVE samples, we establish that prediction error can be explained by differences in distributions of OC and aerosol composition between calibration and test set. This work is an initial step in proposing a non-destructive analysis method that can reduce the operating costs of large air quality monitoring networks.