Tunable diode laser spectroscopic detection of formaldehyde (H ₂ CO) and HCl coupled with laser flash photolysis of Cl ₂ CO–CH ₃ SCH ₃ –O ₂ –N ₂ mixtures, in both the presence and absence of NO, has been utilized to conduct a mechanistic and kinetic investigation of the atmospheric oxidation of the CH ₃ SCH ₂ radical, a product of dimethyl sulfide (DMS, CH ₃ SCH ₃ ) reactions with OH and NO ₃ in the atmosphere. The temperature dependence of the CH ₃ SCH ₂ O ₂ + NO rate coefficient (k ₂ ) and the 298 K rate coefficient for the CH ₃ SCH ₂ O ₂ self reaction (k ₄ ) have been measured. The Arrhenius expression k ₂ = 4.9 × 10 ⁻¹² exp(263/T) cm ³ molecule ⁻¹ s ⁻¹ adequately summarizes our CH ₃ SCH ₂ O ₂ + NO kinetic data over the temperature range 261–400 K. Contributions from side reactions, which are not completely quantifiable, limit the accuracy of the k ₄ (298 K) determination; our results indicate that the true value for this rate coefficient is within the range (1.2 ± 0.5) × 10 ⁻¹¹ cm ³ molecule ⁻¹ s ⁻¹ . In both reactions CH ₃ SCH ₂ O ₂ is converted to H ₂ CO with unit yield (at T = 298 K). Our results demonstrate that the lifetime of CH ₃ SCH ₂ O, a proposed precursor to H ₂ CO, is less than 30 µs at 261 K and 10 Torr total pressure.