2,2-dimethyl-5-[3-(4-methylphenyl)-2-propenylidene]-1,3-dioxane-4,6-dione, C16H16O4, crystallizes in the triclinic space group P-1, with two molecules in the asymmetric unit. These molecules and a centrosymmetrically related pair, linked together by weak C-H...O hydrogen bonds, form a tetramer. 5-[3-(4-Chlorophenyl)-2-propenylidene]-2,2-dimethyl-1,3-dioxane-4,6-dione, C15H13ClO4, also crystallizes in the triclinic space group P-1, with one molecule in the asymmetric unit. Centrosymmetrically related molecules are linked together by weak C-H...O hydrogen bonds to form dimers which are further linked by yet another pair of centrosymmetrically related C-H...O hydrogen bonds to form a tube which runs parallel to the a axis.