This work focused on the monitoring during the roasting process of one of the main characteristics of coffee's organoleptic profile: acidity. Batches of Arabica and Robusta coffee varieties from different origins were roasted following different process conditions. Simultaneously, real-time on-line monitoring of the coffee roasting process was performed with near infrared spectroscopy (NIRS) using a diffuse reflectance probe. Spectral data were analysed with chemometric tools and acidity profiles were estimated directly from NIR spectra, revealing an encouraging agreement with values obtained with the reference analytical methodology. NIRS proved to be a reliable non-invasive technique for real-time monitoring of coffee roasting processes and may be used as an end-roasting signalizing tool. Furthermore, the approach presented also revealed good perspectives for the exploration of NIRS in the monitoring of other relevant compounds during coffee roasting.