Nanoparticles and sub-microparticles of the conducting polymers polyaniline (PANI), polyanisidine (PoANIS) and their copolymers were synthesised, deprotonated and dispersed in viscous media in order to study the influence of their synthetic conditions and of steric stabilisers (cellulose-based materials) on the physical, chemical and morphological characteristics of the polymers. Electrorheological (ER) measurements were performed and related to the polymer properties. The polymer particles had various loadings of stabilisers, depending on the polymer/stabiliser interactions and the stabiliser concentration. When stabilised with hydroxyethylcellulose (HEC), the PANI particles contained traces of HEC, while PoANIS, less stabilised by the electron-rich HEC molecules, did not. Stabilised PANI-HEC synthesised at 0°C readily formed small fibres that, when deprotonated, displayed a large electrorheological response (yield stress ca. 800 Pa at 3.2 kV.mm−1). PoANIS prepared under the same conditions yielded a polydisperse cenospheric material with no ER activity.