Solidification-related phenomena and the properties of the final product are strongly influenced by the developing dendritic microstructure, which is defined e.g. by the secondary dendrite arm spacing. In the past, different experimental set-ups were applied and subsequently the secondary dendrite arm spacing of certain steel grades was measured. However, it is difficult to compare the proposed relations based on either the local solidification time or the cooling rate, and they also vary over a wide range. Therefore, the present study systematically investigates the effect of carbon on the secondary dendrite arm spacing using in situ solidification experiments with accurately defined solidification conditions. The parameter K in the empirical equation lambda(2)=K.t(f)(1/3) was determined as a function of carbon, using an iterative procedure to calculate the local solidification time and the measured secondary dendrite arm spacings. Furthermore, these results were discussed and compared with theoretical models from the literature.