The two-pulse echo sequence is examined for the case in which the frequency modulation time tau(m) of the transition is intermediate between the well known limiting cases of very fast modulation (motional narrowing) and very slow or static modulation (inhomogeneous broadening). Within this spectral diffusion regime, the interpretation of the echo decay differs markedly from standard treatments. If the frequency-frequency correlation function initially decays as 1-t(beta), the echo decay time T-E is proportional to tau(m)(beta/(beta+2)). These results reduce to those of Yan and Mukamel [J. Chem. Phys. 94, 179 (1991)] for beta = 1. Drawing on a viscoelastic model, the theoretical results are compared to viscosity and temperature dependent vibrational echo experiments on myoglobin-CO. A tau(m)(1/3) dependence is observed, as is predicted for an exponential decay of the frequency-frequency correlation function.