Dependencies of the radii of expanding spherical flames on time elapsed after spark ignition have been measured using a high-speed Schlieren technique in a constant volume bomb. These dependencies are analyzed in order to find the Markstein numbers. Various methods of finding different Markstein numbers are considered. Phenomenological Markstein numbers with respect to the combustion products are obtained by means of the comparison between the measurements of the flame radii as a function of time and the results of the analytical integration of the linear relation between the dame speed and either flame stretch rate or flame curvature. The linear equation with respect to the flame stretch rate cannot approximate the experimental data corresponding to high flame curvature, while the linear equation with respect to the flame curvature approximates all the experimental data with a satisfactory accuracy. The correctness of this linear relation for highly curved flames and the effect of different data fitting schemes are briefly discussed.