This dataset contains the underlying data associated with the manuscript entitled "Photofragment Translational Spectroscopy Studies of H Atom Loss Following Ultraviolet Photoexcitation of Methimazole in the Gas Phase" Experimental data All of the raw data files are presented as HRA-PTS ToF spectra; the time step between each data point is 0.01us, with the first data point at 10.12us. Spectra were in general recorded in multiple runs, usually of ca. 25000 repetitions each, which were then averaged together and converted to TKER space. The flight distance used for this conversion was calibrated using spectra recorded from the photodissociation of hydrogen sulfide when the laser alignment or experimental conditions changed. The spectra used to produce the data published in figures 4 and S2, and the calibrated flight distance, are: 275nm: Average of mmi_275nm_a, mmi_275nm_b, and mmi_275nm_c (15000 cycles each; flight distance 0.6100m) 270nm: Average of mmi_270nm_a, mmi_270nm_b, and mmi_270nm_c, mmi_270nm_d (25000 cycles each; flight distance 0.6100m) 267.5nm: Average of mmi_267-5nm_a, mmi_267-5nm_b, and mmi_267-5nm_c (25000 cycles each each; flight distance 0.6100m) 265nm: Weighted average of a mmi_265nm_a (50000 cycles), mmi_265nm_b, and mmi_265nm_c (25000 cycles each; flight distance 0.6083nm) 260nm: Weighted average of mmi_260nm_a (35000 cycles), mmi_260nm_b (17750 cycles), mmi_260nm_c, and mmi_260nm_d (25000 cycles each; flight distance 0.6103m) 255nm: Average of mmi_255nm_a, mmi_255nm_b, and mmi_255nm_c (25000 cycles each; flight distance 0.6103m) 250nm: Average of mmi_250nm_a, mmi_250nm_b, and mmi_250nm_c (25000 cycles each; flight distance 0.6103m) 245nm: Weighted average of mmi_245nm_a (15000 cycles), mmi_245nm_b, mmi_245nm_c, mmi_245nm_d, and mmi_245nm_e (25000 cycles each; flight distance 0.6103m) 240nm: Average of mmi_240nm_a, mmi_240nm_b, mmi_240nm_c, mmi_240nm_d, and mmi_240nm_e (25000 cycles each; flight distance 0.6103m) 235nm: mmi_235nm (25000 cycles; flight distance 0.6103m) 230nm: Average of mmi_230nm_a, mmi_230nm_b, mmi_230nm_c, mmi_230nm_d, and mmi_230nm_e (25000 cycles each; flight distance 0.6103m) 225nm: Average of mmi_225nm_a and mmi_225nm_b (15000 cycles each; flight distance 0.6100m) 222.5nm: Average of mmi_222-5nm_a and mmi_222-5nm_b (15000 cycles each; flight distance 0.6100m) In addition, the data for figure S3 were taken at each wavelength as a series of HRA-PTS ToF spectra recorded for 1000 cycles each, alternating between parallel and perpendicular polarisation of the photolysis laser. These are included as files with the prefixes mmi_beta2_230nm_par and mmi_beta2_265nm_par for the parallel polarisation spectra, and mmi_beta2_230nm_perp and mmi_beta2_265nm_perp for those at perpendicular polarisation. The flight distances were 0.6103m for the 230nm experiment, and 0.6100m for the 265nm experiment. The UV/Vis spectral data for figure 1 are also available in MMI_UV.CSV, which contains the absorbance vs wavelength spectral data in a comma-separated format. Computational data These data are split into the following directories: CASPT2-PE-Profiles - contains the ground and excited state unrelaxed potential energy profiles associated with the N-H and S-H stretch coordinates of the thione and thiol tautomers, respectively, for figure 3 CASPT2-LIIC - contains the ground and excited state linear interpolated paths connecting the ground state minimum energy geometry with the lowest energy triplet minimum energy geometry (liic-*.out) or the ground state minimum energy geometry with the minimum energy geometry of the first electronically excited singlet state (point-*.out), shown in figure 7 CI-Optimisation - contains the data set associated with the minimum energy conical intersection between the ground and S1 excited state seen in figure 7 Excited-State-Optimizations - contains the data sets for the optimization of the radical excited states (radical*.log) and the parent thione tautomer excited states (s*.log) Vibrational-Frequencies - contains the output files for the anharmonic frequencies calculations for the thione and thiol tautomers of the parent molecule and for the radical, seen in tables S1-S3 All files are either MOLPRO or Gaussian 09 output files, with .out and .log suffixes respectively.