A very efficient chemical pre-treatment method that uses cheap and safe hydrated-salts as catalysts for the conversion of waste cooking oils and animal fats having a high Free Fatty Acids (FFAs) content into an oily feedstock convertible into biodiesel through a conventional route (basic catalysis) was investigated. These hydrated-salts allow FFAs to be efficiently (>99%) esterified with alcohols under very mild conditions (343 K, 2 h). At the end of this treatment, a very convenient separation of products was verified. Two different phases were eventually obtained: an upper alcoholic phase, which contains most of the unreacted alcohol, water obtained by direct-esterification (>95%), and the salt that was used as catalyst (recovery >99%); and a lower oily-phase mainly composed of glycerides, methyl-esters derived from direct-esterification of FFAs (residual acidity of about 0.8 mg KOH/g), and part of unreacted alcohol (7-10%wt). Such a separation was convenient because the oily-phase could be directly trans-esterified through conventional base-catalysis, without any further pre-treatments, thus avoiding production of salty-waste. Also, the alcoholic phase could be recovered and directly re-used for new pre-treatments cycles of fresh waste-oils. A final scheme of the proposed process was discussed and the relevant advantages with respect to conventional routes were highlighted. (C) 2019 Elsevier Ltd. All rights reserved.