The purpose of this work was to study the anaerobic co-digestion of all the by-products generated during the processing of used vegetable oils with the objective of proposing a solution for the treatment of these wastes. More specifically, this study investigates the advantages of two-phase anaerobic digestion for treating a mixture (1/5 v/v) of used vegetable oils processing wastes (OW) and pig manure (PM) using two semi-continuous digesters operated at mesophilic temperature (37±1 ºC). The experiments were conducted at hydraulic retention time (HRT) of 0.5, 1, 2 and 4 days in the first stage (acidifier) and at HRT of 11.5, 15, 18 and 20 days in the second stage (methaniser). The results revealed that the HRT had a high influence on the soluble chemical oxygen demand (sCOD) and total dissolved solids (TDS) removal. The maximum total sCOD removal efficiency of 86.4% and TDS removal efficiency of 81.9% was achieved at 20 days of global HRT. The maximum biogas production of 0.65 m3 per kilogramme of volatile dissolved solids (VDS) removed, corresponding to a methane production of 0.42 m3CH4 kg-1 VDS removed (65% CH4) was also achieved at 20 days of HRT. The two-phase digestion system showed good stability, which was mainly attributed to the strong buffering capacity with the two-phase system and the high alkalinity from PM when co-digested with OW. The results obtained from this study provide fundamental information for scaling up a high-performance two-phase anaerobic system in the future.