A novel fabrication of water-dispersible polyaniline-poly(4-styrenesulfonate) (PANI-PSS) nanoparticles with an average diameter of ca. 30 nm is presented. PANI-PSS nanoparticles can be easily processed and thus used in inkjet-printed chemical sensors (an inkjet-printed image is shown in the figure). These chemical sensors shows unprecedented sensitivity and rapid response time compared to the conventional PANI-based chemical sensors.