Historic records and research have suggested that the 1783–1784 eruption of the Laki fissure volcano in Iceland impacted Northern Hemisphere climate significantly, probably as a result of the direct injection of volcanic materials into the stratosphere where the volcanic aerosols would linger for years to cause surface cooling across the Northern Hemisphere. However, recent modeling work indicates the Laki climatic impact was limited to the Northern Hemisphere and only in the second half of 1783. We measured sulfur-33 isotope excess (Δ33S) in volcanic sulfate of historical eruptions including Laki found in Summit, Greenland ice cores. No Δ33S excess is found in sulfate of apparently tropospheric eruptions, while sulfate of stratospheric eruptions is characterized by significant Δ33S excess and a positive-to-negative change in Δ33S during its gradual removal from the atmosphere. Because the same characteristics have been previously found in volcanic sulfate in Antarctica snow, the results from Greenland indicate similar global processes of stratospheric chemical conversion of SO2 to sulfate. The isotopic composition of Laki sulfate is essentially normal and shows no characteristics of sulfate produced by stratospheric photochemical reactions. This clearly indicates that the Laki plume did not reach altitudes of the stratospheric ozone layer. Further, the short aerosol residence time (<6 months) suggests that the bulk of the Laki plume and subsequent aerosols were probably confined to the middle and upper troposphere. These conclusions support the hypothesis of D'Arrigo and colleagues that the unusually cold winter of 1783–1784 was not caused by Laki.