The paper examines the correctness of the best-known approximate methods of interconversion between the frequency- and the time-dependent material functions. Approximate interconversions are compared to the close-form viscoelastic interrelations through the relaxation and/or retardation spectra. The analysis showed that the simple method, G(t) similar or equal to G '(omega)vertical bar(omega=1/t), should be avoided. The most successful was the method of Schwarzl yielding results with maximum relative error within 2%. The magnitude of the error of interconversion is related to the magnitude of the response function first derivative. When the first derivative psi = vertical bar d(log G '(omega))/d(log(omega))vertical bar < 0.5 all approximate methods except the simple method furnish interconversions within the acceptable error, i.e., less than 5%. (c) 2005 Elsevier B.V. All rights reserved.