The effect of CO2 on the morphology of polyaniline (PANI) polymerized in the presence of compressed CO2 was investigated. Two dissimilar morphologies, nanorods and nanoparticles, were successfully obtained using polymerization with and without compressed CO2. The obtained nanorods and particles were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Brunauer, Emmett and Teller (BET) analysis, and scanning electron microscopy (SEM). FT-IR and XRD results showed the presence of polyaniline emeraldine salt form. The SEM images clearly showed PANI nanorods (PANNRs) and nanoparticles (PANNPs) in polymerized PANI. From SEM images, the average diameters of PANNPs and PANNRs were calculated to be 85 and 90 nm, respectively. BET analysis indicated the surface area of PANNRs to be 44% larger than that of the PANNPs. The adsorption performance of the prepared PANI was evaluated in a decolorization process of acid blue 25 dye (AB25). The PANNRs always showed better adsorption compared with PANNPs. The PANNRs recorded a higher decolorization rate, with 99.8% of the dye adsorbed within 8 min; whereas, PANNPs showed a rate of 99.38% decolorization in 16 min. The adsorption kinetics of both PANI were analyzed with both Freundlich and Langmuir models. As a result, the Langmuir isotherm model showed better agreement with the experimental data of the dye adsorption process than that of Freundlich model. Furthermore, it was found that the dye adsorption procedure onto the two fabricated PANI structures follows the pseudo-second-order model.