In this study, two olive mill wastes - exhausted olive cake (EOC) and paste of olive mill wastewater naturally dehydrated (POMW) were co-composted and mixed with 25% sesame bark (SB). The humification process was evidenced by quantifying the humic substances and the generally accepted humification indices: (i) the ratio of humic acid (HA) carbon to fulvic acid (FA) carbon (CHA/CFA), (ii) the ratio of water soluble organic carbon (C-W) to total organic nitrogen (C-w/N-org), (iii) and the ratio of humic acid carbon to total organic carbon C-HA/C-org and by determining the absorbance ratios: E-2/E-4, E-2/E-6 and E-4/E-6. The results showed that the time required to reach maturity was dependant on the chemical properties of the initial raw materials used. The compost including EOC had more nitrogen and synthesised more polymerised HA, the POMW compost also had acceptable degrees of stability and maturity at the end of the process. Maturation was confirmed by a decline in C-w below 1.7, an increase in nitrogen, in HA, in C-HA/C-FA and an elimination of phytotoxicity. Composts produced with olive mill wastes, experimented on potato culture in the field, can be considered beneficial to soils because of their humification indexes and no toxicity. (C) 2008 Elsevier Ltd. All rights reserved.