Two flow selection interfaces have been developed for connecting single or linked reactors with on-line SFC equipped with UV/Vis diode-array (DAD) and mass spectrometer detectors coupled in series (SFC-DAD-MS). This process analytical technology system, enabled the study of reactions performed in supercritical fluid carbon dioxide (SF-CO2). Each interface is designed to obtain on-line samples at different reaction progress points. Three single stage electrophilic annulation reactions performed in SF-CO2 using aniline, indole and pyrrole as amine reactants to synthesize N-heterocyclic products within a stirred continuous flow reaction vessel containing montmorillonite K-10 catalyst were monitored. Also, both stages of a two step synthetic procedure conducted within two connected continuous flow reaction coil systems were monitored using on-line SFC-DAD-MS. The first step involved a Knoevenagel reaction in SF-CO2 that provided a continuous feed to perform a second stage acetylation reaction. Use of on-line SFC-DAD-MS enabled early detection and characterization of unexpected by-products.