We investigate spatial temperature fluctuations in the unburned endgas of a spark-ignited (SI) engine. Our approach exploits the fact that the quantities (species concentrations, temperature) comprising the state of the combustion chamber gases are not independent, but often correlate sharply. Information about spatial temperature variations is obtained indirectly by observing two complementary quantities that sensitively reflect the local temperature. Spatial formaldehyde (H2CO) concentration variations in the endgas are converted to temperature fluctuations using calculated correlations between temperature and [H2CO]. Additionally, the geometrical size of inhomogeneities is deduced from LIF-images displaying the locally varying formation of temperature-sensitive intermediate species. The results indicate that temperature fluctuations exceeding 20 K are present in the nominally homogeneous endgas. The geometrical size of these fluctuations ranges from approximate to1 mm to approximate to1 cm. (C) 2003 The Combustion Institute. All rights reserved.