Until recently, the heats of formation of some of the simplest adsorbed molecular fragments such as -CH, -CH2, -CH3, -OH, -OCH3, -OOCH and –C(CH3)3 were not known on any metal surface. However, these simple adsorbates are ubiquitous catalytic reaction intermediates involved in many industrially important reactions that are catalyzed by platinum-group metals, including water-gas shift, steam reforming of hydrocarbons and oxygenates, combustion and selective oxidation reactions of all sorts, methanation and Fischer-Tropsch, methanol synthesis and decomposition, several fuel cell reactions and photocatalytic water splitting. Here we summarize the first measurements of the heats of formation of these adsorbed intermediates on Pt(111) using single crystal adsorption calorimetry. We also use these heats of formation to predict reaction energies for important elementary steps in catalysis on Pt, which give insights into catalytic reaction pathways. The results will also be compared to Density Functional Theory (DFT) calculations, to assess which DFT methods are most accurate. These experimental benchmark energies are aiding in the ongoing development of more accurate computational methods, like DFT with van der Waals corrections.