Ethoxylation and propoxylation reactions are often performed together or in alternation to obtain surfactants with particular properties or random and block copolymers. Both reactions are normally performed in the same reactor, in the presence of an alkaline catalyst, at relatively low temperature, 120-130 degrees C, to avoid the intervention of the side reactions that are typical of the propoxylation. The propoxylation of a primary fatty alcohol is slower than the corresponding ethoxylation and gives place to a secondary hydroxyl terminal group that is still less reactive. On the contrary, ethoxylation restores more reactive primary hydroxyl terminal groups. Therefore, it is important for optimizing the described industrial operations to know the reactivity of ethylene and propylene oxide with, respectively, primary and secondary hydroxyls, in the presence of the most used KOH catalyst. In this paper, the kinetics of both the ethoxylation and propoxylation of 1- and 2-octanol catalysed by KOH have been studied for this purpose. We will show, first of all, that propylene oxide ring opening occurs selectively giving only secondary hydroxyls as terminal groups. The ratio of addition rate of ethylene oxide to primary and secondary alcohol with respect to that of propylene oxide is always greater than 1. Kinetic data collected have been interpreted by using a kinetic model able to simulate during the time the consumption of both the octanol and the alkoxide and the evolution of the oligomer distributions. The kinetic model and related parameters can be easily extrapolated to different industrial situations in which ethoxylation and propoxylation occur together or in alternation.