To provide a Lewis acid catalyst composition containing a medium making improvement in the reaction efficiency, separation of a product, and recovery and reuse of a catalyst easy in performing the reaction using a Lewis acid catalyst, and to provide a recovery method for the Lewis acid catalyst using the Lewis acid catalyst composition.

The Lewis acid catalyst composition contains a carbon dioxide medium (A) and at least one specific Lewis acid catalyst (B) selected from sulfonylimide metal salt and sulfonylmethide metal salt. The recovery method for the Lewis acid catalyst has a reaction process for reacting the Lewis acid catalyst composition with a reactive substance in a supercritical state of the carbon dioxide medium (A) in the Lewis acid catalyst composition, and a process for separating into the Lewis acid catalyst (B) and liquefied carbon dioxide containing the reaction product and purifying them after the carbon dioxide medium (A) in the Lewis acid catalyst composition is converted into the liquefied carbon dioxide from the supercritical state.