A process for the pretreatment of solid wastes, and wastes to be compacted, for introduction into underground salt cavities or salt caverns via a downpipe under the force of gravity, wherein solid, uncomminuted or comminuted wastes are mixed with waste sludges in a ratio of solid wastes to waste sludges of from 60:40 to 95:5, based on the sum of the substances as 100% by weight, and the water content of the dry wastes is adjusted by the addition of an aqueous salt solution to from 1 to 15% by weight, based on the total amount of dry wastes, and from the resulting material moldings are produced in the form of granules, agglomerates or pellets, and, if appropriate, the moldings are surface-dried or dried completely and are then allowed to flow into the salt cavity or salt cavern and form a pile therein until the said cavity or cavern is substantially full, and, if required, the salt cavity or salt cavern is closed.