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The novel aluminum ethylene diphosphonate Al2(OH)2(H2O)2(O3PCH2CH2PO3) has been synthesized hydrothermally. It crystallizes in the triclinic space group (P1̄; a = 8.5977(10), b = 7.0499(3), c = 9.5290(4) Å; α = 89.997(21), β = 104.321(8), γ = 114.225(11)°; V = 506.98(8) Å3) with aluminophosphonate layers similar to those found in Al(OH)(H2O)CH3PO3 linked via ethylene (-CH2CH2-) groups. The solid loses water upon heating in two steps, the first at 100 °C and the second at 360 °C. The first water loss leaves a crystalline solid with a framework structure of Al2(OH)2(O3PCH2CH2PO3), closely related to that of the parent (C2/c; a = 15.3723(21), b = 6.7613(8), c = 9.7374(13) Å; β = 102.392(9)°; V = 988.4438 Å3) whereas the second water loss results in major loss of crystallinity. X-ray powder diffraction has been used to determine and refine the structures of the parent and intermediate dehydrated phases. The main structural difference is the loss of one water molecule bound to aluminum, which changes the metal cation's coordination from octahedral to distorted square planar. The as-prepared and dehydrated samples have been simulated computationally. The transformation has been monitored by in-situ 27Al MAS NMR spectroscopy; octahedral aluminum (δiso = −7.1 ppm; asymmetry parameter, η = 1.0; quadrupolar coupling constant, QCC = 5.4 MHz) is converted to 5-fold coordinated aluminum (δiso = 20.7 ppm; η = 0; QCC = 5.8 MHz) by heating at 130 °C for 1 h. The process is fully reversible.