Abstract: In an existing isothermal dyeing process, the alkali required for dye fixation is added to the neutral dyebath under linear or progressive program control. In this process, the amount of alkali added per unit time is small at the beginning and is increased progressively and in accordance with a program. This process passes through a certain (non-controlable) initial phase until the start of dye fixation on the cellulose. As a consequence the rate of fixation passes through a maximum before gradually decreasing when the final state of the dyeing is reached. To shorten the initial phase (waiting time) of dye fixation and to slow down dye fixation during the steepest part, it has proved advantageous according to the invention to add the alkali in accordance with a parabolic program.