As a renewable alternative energy source, biomass is currently receiving serious attention as a means to meet energy demands. Palm kernel shell (PKS) has been selected from among other oil palm wastes for thermochemical conversion studies for the purpose of solid/liquid fuel production. In this study, the influence of the washing pretreatment process was observed by treating PKS feedstock with various types of washing medium to observe the impact of treated and untreated feedstock on pyrolysis yield and product characteristics. The washing mediums tested were reverse osmosis (RO), tap water and distilled water. Meanwhile, 5% dilute acetic acid and sodium hydroxide (NaOH) were prepared as mildly acidic and alkaline washing media, respectively. The highest percentage of ash reduction was clearly found in the treated PKS soaked in distilled water, reaching 43.05%. Untreated PKS was pyrolysed between 300°C to 500°C to observe the yield distribution of pyrolysis products, while treated PKS was pyrolysed at 400°C to investigate the influence of ash content on pyrolysis yields and product characteristics. Under the temperature profile, the highest yield of char was obtained at 300°C, amounting to approximately 54.40 wt%; meanwhile, the pyrolysis liquid yield reached its maximum at a temperature of 400°C but then decreased with further increases in temperature. For the treated PKS series, the highest char yield was produced from PKS treated with diluted alkaline, at 46.96 wt%, while the maximum yield of pyrolysis liquid was 46.01 wt% from PKS treated with diluted acid. The energy content in char was seen to increase as the ash content became lower. The presence of ash, which acts as a catalyst, promotes secondary reactions during pyrolysis and is thus considered a significant contributor to high water fractions in the organic liquid yield. Suggestions for improving the quality of pyrolysis products are also discussed in relation to this study.