Viscous friction between mobile and stationary phase can generate a considerable amount of frictional heat. Its extent mainly depends on the column backpressure and mobile phase characteristics. Frictional heat can cause radial and axial temperature gradients within a column. Both, thermostatting mode and column diameter influence the formation of a temperature profile in a column (Fig.1). The still air mode prevents from the well-known efficiency loss by frictional heat which occurs with forced air mode. However, the average column temperature increases as the entire frictional heat remains in the column when the still air mode is used. Thereby, changes in retention and, possibly, in selectivity occur. With the forced air mode, frictional heat dissipates from the column and analyte retention should not change significantly.