Abstract Chars were prepared under standard conditions (750 °C, N 2) from ten different types of biomass belonging to a wide range of plant origins and with a wide range of natural metal ion contents. When these chars were gasified with carbon dioxide at 700 °C the rates of gasification were found to show linear dependence on the combined molar concentration of the dominant metals (potassium and calcium), with correlation coefficient r = 0.970, provided two samples were omitted. The samples omitted were from wheat straw and coir dust, which had the highest silicon content of the samples studied and showed abnormally low rates of gasification. It was concluded that the catalytic effect of K was reduced by reaction with silica to form silicate during pyrolysis. This was confirmed by addition of silica to a high K, low Si sample (potato pulp) and by observation of the reduced gasification rate in the derived char. Catalysis of gasification by Ca (in lignite) does not appear to be significantly reduced by silica under the above conditions. These results indicate that the gasification reactivities of chars from any type of biomass can be predicted with reasonable certainty from knowledge of the metal ion content, provided the biomass has low Si content. It also appears that the chemical and physical properties of the chars are remarkably similar, despite the wide range of plant species and morphologies from which they are derived.