Supercritical water gasification is an efficient technique for gasifying biomass in particular with high moisture content. Therefore, it is a promising technology that allows gasification of microalgae. Supercritical water gasification (SCWG) of three microalgae was conducted at 673 K and 25 MPa, which produces various gas products such as carbon monoxide, carbon dioxide, methane, ethane, ethylene, and hydrogen gas. SCWG was conducted in the presence and absence of nickel catalyst and sodium chloride salt. In the presence of nickel catalyst, microalgae were gasified more efficiently relative to non-catalytic SCWG. Surprisingly, addition of salt to the SCWG seems to increase the carbon gasification efficiency. Also a new approach was studied by addition of catalyst and salt which reveals lower carbon gasification efficiency. SCWG of marine microalgae seems to be more efficient than freshwater microalgae as higher amount of carbon dioxide and hydrogen gas was produced. © 2016 The Authors.