In-operando measurements were carried out for the first time to identify different intermediates and primary products formed during the conversion of lipids for catalytic as well as non-catalytic thermal hydroprocessing. Hydroprocessing of lipids was carried at 280-300 degrees C temperature, 40-60 bar pressure, 1500 Nl/L H-2/Feed ratio and 1 h(-1) space velocity in a fixed-bed tubular reactor. A high pressure metallic probe was used for collecting the FTIR spectra under these conditions. Under these conditions although the conversion of triglyceride molecules were complete, the deoxygenation reactions were less than 1%, with 6-7% carbon loss and 17-18% hydrogen loss into the gaseous phase corresponding to propane. In-operando FT-IR studies confirmed that depropanation (propane removal) is the primary reaction during catalytic hydroprocessing of triglycerides, along with formation of intermediates. For thermal hydroprocessing wide range of intermediates -alcohols, alkenes, ketones, anhydrides, ethers, acids, aldehyde and esters -were observed. Such intermediates indicate several undesirable coupling reactions such as CeC bond formation, esterification, etherification and ketonization. For catalytic hydroprocessing only selective product intermediates were observed. Triglycerides hydroprocessing over Pd/Al2O3 catalyst predominantly produced carboxylic acid and aldehyde intermediates, along with very weak bands due to aromatics. Pd/C predominantly produced aldehydes with weak bands due to ketones. Sulfided CoMoP/Al2O3 catalyst also produced aldehydes predominantly, but also small quantities of carboxylic acids as intermediates, from trigycerides. IR bands due to alcohols, ethers, aromatics and cyclic ketones were observed at the later stage of reaction over CoMoP(S)/Al2O3 catalyst.