The invention relates to a process for preparing an internal olefin sulfonate, comprising sulfonating an internal olefin, having an average carbon number that is equal to or greater than 20, in a sulfonation reactor into sulfonated internal olefin followed by contacting sulfonated internal olefin with a base containing solution, wherein the sulfonation reactor is cooled with a cooling means having a temperature which is greater than 35° C. Further, the invention relates to an internal olefin sulfonate obtainable by said process, and to use of internal olefin sulfonates obtainable or obtained by said process as a surfactant, suitably as a surfactant in chemical Enhanced Oil Recovery.