Activated clay was used to study the adsorption behavior of dyestuffs in synthetic wastewater containing dyestuffs. Three basic dyes were used: C.I. Basic Red 18 (or BR18), C.I. Basic Red 46 (BR46), and C.I. Basic Yellow 28 (BY28). Adsorption occurred almost instantaneously upon contact. The mechanism of adsorption was explained by a charge to the electrostatic attractive force described in the Langmuir adsorption isotherm. The mass transfer coefficient was also calculated by the external mass transfer model in an adsorbent according to Mckay et al. Parameters including species of basic dyes, initial concentration, temperature, size of adsorbent, and NaCl were extensively investigated.