Adsorption of Methylene Blue (MB) and Bromocresol Green (BG) were examined by using biochars produced from rice residues, straw and husk of Japanese Koshihikari and Vietnamese IR50404 rice varieties, to evaluate their chemical adsorption capacity and possible adsorption mechanism. Cationic MB dye were more effectively adsorbed than anionic BG dye by all biochars examined. Vietnamese IR50404 biochars showed higher capacity of adsorption of two dyes than Japanese Koshihikari biochars, approximately about 1.5 times for MB & 1.7 times for BG. In varying pH 2 to 10, alkaline condition increased adsorption of cationic MB dye - increasing by 27.7 - 33.5% for rice straw biochars & by 86.2 - 92.2% for rice husk biochars; albeit slight decrease in adsorption of anionic BG dye by  27.2 - 32.9% for rice straw biochars & by 47 -70.6% for rice husk biochars. The pH experimental results indicate the existence of negative charges in the biochars and their electrostatic interaction with dyes. The adsorption kinetic study supported intra-particle diffusion of dyes, proceeding via a complex mechanism consisting of both surface adsorption and intra-particle transport within the pores of biochars. Thermodynamic analysis of adsorption suggested that the process was spontaneous with negative ΔG⁰ values and endothermic with positive ΔH⁰ values (ΔG⁰ = 37.02 – 41.94 kJ/mol; ΔH⁰ = 3.92 - 23.69 kJ/mol). This paper discussed possible explanation of dye adsorption with physisorption through porous diffusion, hydrogen bonding, π-π interaction or π⁺-π interaction, common to both anionic and cationic dyes, with additional electrostatic interaction for cationic dyes with biochars in aqueous solution.