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Diethylamine catalyzed tandem Knoevenagel–Michael reactions have been carried out in aqueous medium as an efficient, greener and cost effective process for the simple one-pot synthesis of bis-dimedone derivatives. Reaction of substituted aromatic aldehyde (1 equiv.) and dimedone (2 equiv.), in the presence of aqueous diethylamine medium at room temperature provides bis-dimedone derivatives 3a–n in excellent yields (87–95%) within a very short reaction time (15–60 min). All the bis-dimedone derivatives were obtained in high purity and the products were fully characterized by physical and spectroscopic data. The structures of compounds 3b,c were elucidated by single crystal X-ray diffraction technique. Compound 3b crystallizes in the monoclinic space group P 21/n with a = 10.2895(9) Å, b = 18.0995(15) Å, c = 15.8615(13) Å, α = 90°, β = 107.975(2)°, γ = 90°, V = 2809.8(4) Å3, and Z = 4. Compound 3c crystallizes in the monoclinic space group P 21/n with a = 10.2816(16) Å, b = 18.080(3) Å, c = 15.637(2) Å, α = 90°, β = 107.076(4)°, γ = 90°, V = 2778.6(7) Å3, and Z = 4. The use of inexpensive, eco-friendly and readily available reagents, easy work-up, and high purity products makes the procedure a convenient and robust method for the synthesis of tandem Knoevenagel–Michael adducts.