IR spectroscopy has been an important tool for studying detailed interactions of reactants and reaction-intermediates with catalyst surfaces. Studying reactions in water is, however, far from trivial, due to the excessive absorption of infrared light by water. One way to deal with this is the use of Attenuated Total Reflection spectroscopy (ATR-IR) minimizing the path length of infrared light through the water. Moreover, ATR-IR allows for a direct comparison of reactions in gas and water on the same sample, which bridges the gap between separate catalyst investigations in gas and liquid phase. This tutorial review describes recent progress in using ATR-IR for studying heterogeneous catalysts in water. An overview is given of the important aspects to be taken into account when using ATR-IR to study heterogeneous catalysts in liquid phase, like the procedure to prepare stable catalyst layers on the internal reflection element. As a case study, CO adsorption and oxidation on noble metal catalysts is investigated with ATR-IR in gas and water. The results show a large effect of water and pH on the adsorption and oxidation of CO on Pt/Al(2)O(3) and Pd/Al(2)O(3). From the results it is concluded that water affects the metal particle potential as well as the adsorbed CO molecule directly, resulting in higher oxidation rates in water compared to gas phase. Moreover, also pH influences the metal particle potential with a clear effect on the observed oxidation rates. Finally, the future outlook illustrates that ATR-IR spectroscopy holds great promise in the field of liquid phase heterogeneous catalysis.