The objective of this study was to investigate the influence of the ash content of oil palm empty fruit bunch (EFB) feedstock on the biochar yield percentage produced by slow pyrolysis experiments. The characteristics of the biochar produced were also investigated. Proximate analysis, elemental analysis and lignocellulosic analysis of EFB feedstock were performed. High heating values (HHVs) were determined using a bomb calorimeter. The ash content of the EFB feedstock was varied by washing. The properties of the washed and unwashed EFBs were analysed via thermogravimetric analysis (TGA) for thermal degradation behaviour. EFB feedstocks in the range of 1.60 to 5.29 mf wt% ash content were pyrolysed at 550°C with 1 h residence time. The heating rate was set at 5°C min–1. The biochars produced by slow pyrolysis experiments were analysed by proximate, elemental, scanning electron microscopy (SEM) and BrunauerEmmett-Teller (BET) surface area analyses. The EFB feedstock with the lowest ash content (1.60 mf wt%) produced the lowest biochar yield percentage (23.05 wt %) with the highest fixed carbon percentage (88.34 mf wt%). Reduction of the EFB feedstock ash content from 5.29 to 1.60 mf wt% produced significant changes in the morphological structure of the resulting biochar. The maximum BET surface area of the EFB biochar, obtained from the EFB feedstock with 2.21 mf wt% ash content, was found to be 11.1200 m2 g–1.