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The electrical resistances of powdered samples of polyaniline and poly(o-toluidine) were measured as a function of pressure using a diamond anvil cell up to 22 GPa at room temperature. Since the primary influence of pressure is sample compression, application of pressure leads to an initial decrease in the resistance (enhanced conductivity) of both samples, indicating greater interchain coherence due to reduced interchain separation. Poly(o-toluidine) is seen to have higher resistance values compared with polyaniline for all pressure values recorded, consistent with the fact that it has greater disorder due to the substitution of the larger methyl group on the polymer backbone. Resistance versus pressure measurements recorded during the loading and unloading cycle show asymmetry, suggesting irreversible structural changes in both samples. The crystallinity in poly(o-toluidine) as deduced from our resistance-pressure data is seen to be slightly less than that for polyaniline. (C) 1998 Elsevier Science S.A. All rights reserved.