The spectra presented correspond with the research article entitled “Kinetics of Formic Acid Decomposition in Subcritical and Supercritical Water – A Raman Spectroscopic Study” . Data set contains in situ Raman spectra of the quenched effluent stream, which includes varied concentrations of formic acid, water, CO, CO2, and H2 as reaction products. Each spectrum is collected downstream of the subcritical or supercritical water gasification of formic acid, which occurs at a specified temperature, residence time, a constant pressure of 25 MPa, and a constant initial feedstock concentration of 3.6 wt% formic acid. Additionally, calibration spectra of formic acid in water, and spectra of pure carbon dioxide and high concentration formic acid are provided for model development. Finally, a MATLAB code used for baseline subtraction of raw data files is included with the dataset. The full dataset is hosted in Mendeley Data, https://doi.org/10.17632/hjn8xwskng.1 .