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A facile and efficient protocol for the synthesis of 9-aryl/alkyl-3,3,6,6-tetramethyl-1,2,3,4,5,6,7,8-octahydroxanthene-1,8-diones has been developed by one-pot reaction of dimedone with aldehydes in the presence of InCl₃ or P₂O₅ under solvent-free conditions. The optical behaviors of the xanthenediones have been investigated by UV–vis and fluorescence spectroscopy. The present approach offers the advantages of simple methodology, clean and mild reaction, high atom-economy, short reaction time, low environmental impact, wide substrate scope, high yield, and excellent purity.