The measurement of SO2 levels in vehicle exhaust can provide important information in understanding the relative contribution of sulfur and sulfate from fuel vs. oil source to PM. For this study, a differential optical absorption spectrometer (DOAS) that can measure SO2 down to 20 ppbV in real-time was built and evaluated. The DOAS consisted of an extractive sampling train, a cylindrical sampling cell with a single-path design to minimize cell volume, a spectrometer, and a deuterium lamp light source with a UVC range of ∼200-230 nanometer (nm). Laboratory tests showed detection limits were approximately in the range of 12 to 15 ppbV and showed good linearity over SO2 concentration ranges of 20 to 953 ppbV. Interference tests showed some interference by NO and by NH3, at levels of 300 ppmV and 16.6 ppmV, respectively. The software was modified to a allow for measurements in a narrower spectral range from 206 to 214 nm, when the DOAS interference SO2 response was less than 1.5 ppbV with 300 ppmV of NO injected with, and was slight less than 8 ppbV with 16.6 ppmV of NH3 injected. The DOAS was evaluated using exhaust from a vehicle. The results showed no appreciable interferences on the SO2 measurement for tests on a diesel truck, with very low NH3 emissions, demonstrating the ability of the instrument to measure at 20 ppbV levels without interferences in a low NH3 environment. The gasoline vehicles, on the other hand, had higher levels of NH3, and showed a significant interference, especially for NH3 values exceeding 11 ppmV. For vehicles with higher levels of NH3, an NH3 scrubber such as permeation dryer would likely be needed to scrub NH3 levels to <10 ppmV, as well as some additional updates to the software that have recently been implemented.