Abstract: Stable HCl-crude oil emulsion and its subsequent sludge formation, with detrimental impacts on oil production, may stem from acid stimulation. One major ambiguity in this process is to discern the most influential component of crude oil on the stability of formed emulsions. This fundamental question has not adequately been addressed in previous studies. In this work, the impact of de-asphalted part of crude oil (maltene) has been investigated on the acid-induced emulsion and sludge separately. Accordingly, the emulsion phase separation and the amount of formed sludge have been compared for four crude oils and their maltene samples for different concentrations of ferric ion and acidic pH values. The results of phase separation, as a criterion for emulsion stability, showed that crude oil samples formed 6 to 25 percent more stable emulsions than maltene samples, when using blank HCl. The emulsions of maltene and spent acid (pH = 2) broke completely during the first 15 min after emulsification. In addition, the maltene components usually had less contribution to sludge formation in the presence of blank HCl. It was concluded that asphaltene is the key component during interaction with HCl. However, the maltene of one crude sample formed higher acid sludge in comparison to its crude oil. For acid solutions containing 3000 ppm of ferric ion, the emulsion stability increased for all crude oil and maltene samples. Moreover, the stability of some maltene emulsions increased to 48% and 100% in the presence of 3000 ppm of ferric ions. The presence of ferric ions caused forming very stable emulsions, while most of the sludge formation took place at higher pH values. Finally, it was also attained that emulsion and sludge formations could happen simultaneously.