The CAP-88 environmental radiological assessment computer code was benchmark tested to establish confidence in its results. The results from CAP-88 were compared to the results from the GENII computer code, which has undergone rigorous testing. The codes were benchmarked using 1990 and 1991 monitored atmospheric releases from Idaho National Engineering Laboratory facilities and the results (the effective dose equivalent to the maximally exposed offsite individual) were quantitatively compared using a metric based on the uncertainty in the Gaussian plume model and terrestrial transport models. The results of the benchmark tests were within the 95% acceptance region specified in the test protocol. CAP-88 was found to overpredict effective dose equivalent relative to GENII for elevated releases, largely because CAP-88 calculates a larger atmospheric dispersion factor (chi/Q) than does GENII using the same meteorological data. However, CAP-88 consistently underpredicted effective dose equivalent relative to GENII for ground-level releases. This was because CAP-88 accounts for the processes of plume depletion by dry and wet deposition while GENII does not account for these processes. The effect of depletion was tested and found to be most important for a ground-level release of a highly depositing species such as radioiodine which implies that acceptable benchmark results would be difficult to obtain for a highly dopositing species.