Two new lead nitrilotris(methylene)triphosphonates, Pb[(H 2 O 3 PCH 2 )N(CH 2 PO 3 H) 2 ] I and Pb 2 [(O 3 PCH 2 )N(CH 2 PO 3 H) 2 ]·H 2 O II, have been synthesised. I is triclinic, space group P1 with combining macron]" xmlns="http://www.rsc.org/schema/rscart38">, a=8.5077(2), b=11.2363(3), c=5.9484(2) Å, α=98.802(2), β=104.955(1), γ=80.911(2)°, Z=2. II is monoclinic, space group Pn, a=7.3614(2), b=11.3889(2), c=7.2541(2) Å, β=100.389(2)° and Z=2. Their structures have been solved from laboratory X-ray data by using ab initio powder diffraction methodology. The reliability factors were RwP =9.28%, RP =7.20% and RF =2.67%, for I, and RwP =12.51%, RP =9.38% and RF =4.45%, for II. I shows a new layered structure which has small cavities, inside an inorganic layer, in which the hydrogen phosphonate and the dihydrogen phosphonate groups are located. II is also layered with the water molecule situated in the interlamellar space. Thermal data, IR data and ion-exchange properties are also reported and discussed. II dehydrates to yield orthorhombic Pb 2 [(O 3 PCH 2 )N(CH 2 PO 3 H) 2 ] III. Hydrolysis of I leads to II.