To determine binary diffusion coefficients from tracer response measurements with a coiled capillary column (curved tube) at relatively high flow rates, a conversion formula to correct for the secondary flow effect was derived from the second central moment of the impulse response curve. Its practical expression was obtained analytically by the perturbation method up to lambda(10), where lambda is the ratio of the tube radius to the coil radius. The reliability of the correction was evaluated by an alternative series of lambda. As a result, the effective range of DeSc(1/2) < 8-9.5 without correction widened to DeSc(1/2) < 18-19 to determine the diffusion coefficient within 1% error. Applicability of the correction was demonstrated for phenol and ubiquinone CoQ10 using a coated column, and for benzene using an uncoated column (the Taylor dispersion), in supercritical carbon dioxide at 16-17 MPa and 313.15 K. (C) 2004 Elsevier Ltd. All rights reserved.