The present invention relates to a composition for dyeing keratin fibers, comprising: at least one oxidation dye； at least one nonionic ether of a polyoxyalkylenated fatty alcohol of formula (i) R&mdash；(O-Alk)nOR (i)； in which formula (i): R denotes a linear or branched, saturated or unsaturated C10-C30 hydrocarbon-based radical, R&prime； denotes a linear or branched, saturated or unsaturated C10-C30 hydrocarbon-based radical, which may be substituted with a hydroxyl radical, n is an integer between 1 and 100 inclusive, and Alk represents a linear or branched, preferably linear, (C1-C6)alkylene group such as ethylene or propylene, preferably ethylene； at least one cationic polymer； at least one fatty substance at a concentration of at least 25% by weight relative to the total weight of the composition； at least one chemical oxidizing agent. The present invention also relates to a process using this composition, and to multi-compartment devices that are suitable for implementing the invention.
