We studied the Cl-initiated oxidation of dimethyl sulfide (DMS) at p = 800 Torr and T = 400 - 545 K, using a flow reactor coupled to a microsecond time-resolved tunable vacuum-ultraviolet photoionization mass spectrometer. Following the initiating photolysis laser pulse, we observed complex distributions of transient chemical intermediates and products, which included the peroxy radical RO2 (CH3SCH2O2), the atmospherically important compound hydroperoxymethyl thioformate (HPMTF, HOOCH2SCHO), and 15 other products of competing secondary reactions. We characterized unknown transient chemical species with help from theoretical calculations of ionization energies and cation dissociation channels, demonstrating that HPMTF does not have a stable parent cation and fully fragments into the m/z = 46, 47, 75, and 80 ion channels even at ionization threshold. Similarly, RO2 is detectable solely by the m/z = 61 fragment ion. Using a global time-resolved C and S atom balance tracking approach, we constrained the concentrations of HPMTF and RO2 and determined their absolute photoionization cross sections up to 11.2 eV. The yield of HPMTF increased with higher O2 concentrations and peaked at 70% at T = 475 K and at constant [O2] = 1.5 × 1018 cm-3. Furthermore, we determined the key rate-limiting reaction rate coefficient for HPMTF formation, k3, at 430 - 510 K. Combining our values of k3 with those of Assaf et al. (Assaf, E.; Finewax, Z.; Marshall, P.; Veres, P. R.; Neuman, J. A.; Burkholder, J. B., J. Phys. Chem. A, 2023, 127, 2336-2350) at 314-433 K results in an Arrhenius expression, k3 = (2.44 ± 0.5) × exp (-(7248 ± 78)/T), which is in nearly perfect agreement with the results of Assaf et al. Our characterization of RO2 and HPMTF enables their future quantitative probing in other experimental reactors by photoionization mass spectrometry.