This study proposed a convenient method to determine kinetic parameters of adsorption processes by nonlinear regression of the pseudo-first-order (PFO), pseudo-second-order (PSO), and pseudo-nth-order (PNO) equations with the help of commercial SigmaPlot 11 software. Adsorption characteristic curves were plotted to explain the relationships between PFO/PSO and PNO equations. Activated carbons with a BET surface area of 1461 m2 g−1 and an area fraction due to micropores of 0.95 were prepared by soaking the char derived from betel trunk in concentrated KOH solutions with a KOH/char ratio of 3. The adsorption kinetics of 3 dyes (methylene blue, basic brown 1, acid blue 74) and 4 phenols (2,4-dichlorophenol, 4-chlorophenol, 4-cresol, phenol) from water on the activated carbons were examined to check the applicability and validity of our proposal. The orders for 4 phenols and acid blue 74 were between 1 and 2; that is, the best-fit curves lay between the PFO and PSO curves. The orders of methylene blue and basic brown 1 were higher than 2. The present results indicated that the SigmaPlot software was an efficient tool to obtain the best-fit kinetic parameters of adsorption processes by nonlinear regression.