We present C2--C6 nonmethane hydrocarbon (NMHC) measurements from canister samples obtained in the extratropical lower stratosphere during the fall (November/December 1995), winter (March 1997), and summer seasons (July 1998) as part of the stratosphere-troposphere experiments by aircraft measurements campaign. The flights were carried out from Amsterdam (Netherlands, 52¿N, 4.5¿E) during fall, from Kiruna (Sweden, 68¿N, 20¿E) during winter, and from Timmins (Canada, 48.2¿N, 70.3¿W) during summer. The NMHC measurements have been evaluated along with concurrent in situ measurements of acetone (CH3COCH3), CO, O3, N2O, and CFC-12 (CCl2F2). The vertical distributions of NMHC and acetone as a function of O3 and potential temperature in the lowermost stratosphere show a strong seasonality. Enhanced concentrations of NMHC + CH3COCH3 were found during July up to potential temperatures of Θ = 370 K, whereas during March this was limited to Θ = 340 K, in agreement with stronger isentropic cross-tropopause transport during summer. Increasing methyl chloride (CH3Cl) concentrations with altitude were measured during July, pointing to mixing at the subtropical tropopause. During summer and fall, mean NMHC + acetone concentrations were more than a factor of 2 higher than that during winter. Box model calculations indicate that the observed acetone levels of 0.5--1 ppbv can explain 30--50% of the enhanced OH radical concentrations in the summertime lowermost stratosphere. Using mass balance calculations, we show that a significant tropospheric fraction (≤30%) was present up to Θ = 370 K in the summertime lowermost stratosphere. During winter, the tropospheric fraction approached zero at about Θ = 350 K. The time between selected troposphere-to-stratosphere mixing events and the aircraft measurements has been estimated at 3--14 days. Our results emphasize that isentropic cross-tropopause transport can be a fast process occurring on timescales of days to weeks.