In this article a continuous-discrete high-gain observer is presented to estimate the molar compositions in a distillation column for the Ethanol-Water mixture, by using the measurements of the temperatures in its plates. One of the main advantages of this observer is its constant gain, which is why its tuning depends only on choosing constant parameters that must satisfy some simple algebraic inequalities. The continuous-discrete observer presents a good pursuit of the process dynamics in spite of using less data than a purely discrete observer. The effectiveness of the proposed method is demonstrated using experimental data.