An algorithm is described which allows relaxation line spectra to be interconverted into retardation line spectra, and vice versa. The first line spectrum is generated from a given experimental response such as the relaxation modulus, the storage modulus, or the loss modulus, or from a response such as the creep compliance, storage compliance, or loss compliance. The interconversion algorithm is here applied to the standard linear solid and liquid models, and to a 32-line spectrum simulating the behavior of an entangled polymer.