  We study the solution of the diffusion equation for Ultra-High Energy Cosmic
Rays in the general case of an expanding universe, comparing it with the well
known Syrovatsky solution obtained in the more restrictive case of a static
universe. The formal comparison of the two solutions with all parameters being
fixed identically reveals an appreciable discrepancy. This discrepancy is less
important if in both models a different set of best-fit parameters is used.
