This paper presents a demonstration of the visualization and the characterization of gas–liquid mass transfer in a small bubbly column. The aim is to show how simple experiments can be used to directly quantify mass transfer without the need of sophisticated probes or complex titrations. The method here proposed is based on the “red bottle” reaction, i.e., the reversible oxidation by oxygen of dihydroresorufin to red resorufin. This reaction produces a distinctive red color, and in the specific condition identified, the time required to turn from colorless to red solution can be directly correlated with the inverse of the volumetric mass transfer coefficient kLa (s–1). Therefore, this reaction can be used to estimate the mass flux of oxygen transferred from the gas phase (bubbles) to the liquid phase and to study how this flux is affected by geometrical and operating parameters of bubbly flows. The lab work described is thus very convenient for education purposes. This is the first visual demonstration allowing mass transfer to be quantified with good precision, opening up a wide range of applications.