A model describing the transport of HCl and HFeCl4 from a water phase to a crude oil phase and subsequent sludge formation is proposed. Crude oil phase transfer compounds (PTCs) that are basic in nature facilitate transport of acid to the crude oil phase. These crude oil PTCs are able to migrate to the acid-oil inter-face and form acid-base complexes that can return to the oil phase. Once in the crude phase, these protonated compounds have a propensity to form aggregates. Growth of small aggregates into larger aggregates eventually generates a precipitate known as acid-sludge. Under certain conditions the model predicts the amount of acid-sludge formed to be proportional to the acid activity, or acidity function H-0, in the water phase. This relation was confirmed experimentally. Furthermore, the total amount of phase-transported acid is proportional to the base content of the crude oil. In strong HCl formulations containing Fe3+, the acid HFeCl4 is formed in small amounts that can be transported to the crude oil phase. In principle, the behavior of HFeCl4 is similar to that of HCl. It was shown that HCl and HFeCl4 compete for basic, or receptor, sites and that exchange between these two acids is reversible. The antisludging agent dodecylbenzenesulfonic acid, DBSA, intervenes through the same mechanism; that is, it competes for receptor sites with HCl and HFeCl4. Exchange between HCl/HFeCl4 and DBSA was also shown to be reversible. (C) 2003 Elsevier Inc. All rights reserved.