This study reports occurrence forms of key ash-forming elements in a defatted microalga, and for comparison, its corresponding raw microalga. Freeze-dried powders of a marine microalga (Nannochloropsis oceanica) were sieved to a size fraction of <75 mu m and used as raw microalga. The raw microalga was then extracted with hexane to remove crude lipids and prepare a defatted microalga. The raw and defatted microalgae were subjected to chemical fractionation analysis, i.e., sequential leaching in H2O, 1.0 M ammonium acetate (NH4Ac), and 1.0 M hydrochloric (HCI) acid. The results demonstrate that, whereas the contents of Fe and Al in the raw and defatted microalgae are extremely low, those of other ash forming elements follow a sequence of Cl > > P > Mg > Na > Ca. Chemical fractionation results suggest that virtually all of the Na, K, and Cl in the raw and defatted microalgae are water-soluble. While majority of P in the two fuels are water-soluble and acid-soluble, most of Mg and Ca are leached in water and NH4Ac solution. As determined via chemical fractionation analysis, lipid extraction leads to the content of water-soluble Ca in the defatted microalga being similar to 69.6% higher than that in the raw microalga counterpart, which is accompanied by a reduction in the amount of Ca leached in NH4Ac solution. Similar trend is also observed for Mg, but to a lesser extent. (C) 2017 Elsevier Ltd. All rights reserved.