Abstract: Azo dyes are environmental pollutants and they are recalcitrant to decolorization by photo degradation and other conventional methods of treatment. The adsorption kinetics, intraparticle diffusion and equilibrium partitioning of Methyl Orange (MO), Methyl Red (MR) and Methyl Blue (MB) onto andropogon sorghum waste biomass was studied. As time increased the amount of the dyes adsorbed increased and equilibrium was reached within 40-50 min. Modification of the adsorbent by carboxymethylation and thiolation enhanced the adsorptive capacity of the adsorbent. The kinetics of sorption was well correlated using the pseudo-second order rate equation. The pseudo second order rate constant ranged from 0.742 (g mg-1-min) for Methyl Orange (MO) on thiolated sorghum to 5.741 (g mg-1-min) for Methyl Blue (MB) on carboxymethylated sorghum. The mechanism of sorption was found to be intraparticle diffusion controlled and the intraparticle diffusion rate constants ranged from 0.0969 min-1 for Methyl Red (MR) on thiolated sorghum to 0.8298 min-1 for Methyl Red (MR) on carboxymethylated sorghum. The boundary layer diffusion effects were calculated. The very high values of correlation coefficients (R2) obtained showed that these models fit the experimental data perfectly. Therefore andropogon sorghum was found to be a good adsorbent for adsorptive decolorization of Azo dyes from waste water.