In this work we demonstrate the use of a quad-plane particle image velocimetry technique in order to investigate the three-dimensional behavior of several important quantities for combustion research such as the flame displacement speed and the flame front curvature. In results from a premixed methane flame stabilized in a diffuser burner, a comparison of three-dimensional and two-dimensional data is made in order to critically analyze the error of the usually performed planar measurements. It is shown that two-dimensional measurements can only give an estimate of the real situation under certain circumstances, such as with mainly spherical structures in the flame and the perfect alignment of both the flame propagation and flow direction to the measurement plane. However, in turbulent flames, this alignment can never be achieved due to fluctuations stemming from turbulence. The application of two crossed planes leads to significant improvements and good agreement with the three-dimensional quantities can be observed, although no perfect match is achieved. Flame displacement speeds ranging from -0.4 s(L) to 4.5 s(L). with a mean of 1.1 s(L) were recorded, but were not correlated with the flame curvature or strain rate. (C) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.