An extensive study of the reaction mechanism of the methanolysis reaction of triazinyl systems of non-polar reactive dyes has been done in a neutral, moderately acidic and strongly acidic medium. In a neutral reaction medium, dichlorotriazines are more reactive than the monochlorotriazines. However. in a moderately acidic medium, as supercritical carbon dioxide, an inversion in the reactivity pattern of monochlorotriazines was observed, which is related to the dye structure. Amines as substituent group attached to the triazinyl ring showed the largest conversion and k values, approaching those values of dichlorotriazine. Protonation of the ring, and stabilization of the positive charge in the triazinyl ring due to the mesomeric effect, explain the reactivity increased of monochlorotriazines. When acids were added to the reaction medium, resulting in an increase of the acid concentration, the reactivity of monochlorotriazinyl dyes was surprisingly much larger than clichlorotriazine; k values increased up to a factor of 54. The largest k was measured for the monochlorotriazinyl dye, SlCl-NH2, showing a value of 3.0 x 10(-3) s(-1), while a value of 3.2 x 10(-4) s(-1) was measured in supercritical carbon dioxide. In a strongly acidic medium, the reactivity of triazinyl reactive dyes is substantially influenced by the basicity of the substituted groups in the reactive group, and in the chromophore. Based on this kinetic study, the dyeing of cotton in supercritical carbon dioxide under acidic conditions seems to be very promising when triazinyl dyes are used. (C) 2006 Elsevier B.V. All rights reserved.