A calcium-containing hydrocarbonaceous material is treated with an aqueous mixture, comprising acetate ion and an alkaline material and having a pH in the range of 3.0 to 5.0, in order to extract at least a portion of the calcium from the hydrocarbonaceous material into the aqueous phase. Acetic acid is a suitable source of acetate ion. Ammonium hydroxide, sodium hydroxide and potassium hydroxide are example alkaline materials.