The crystal structure of hydrothermally formed Al-substituted tobermorite-1.1nm (Ca4.9Si5.5Al0.5O16.3(OH)(0.7). 5H(2)O) has been analyzed using synchrotron radiation powder diffraction data. Crystallographic positions of a zeolitic Ca atom and three water molecules in the channel of the framework were determined by the Monte Carlo method and subsequent Rietveld refinement. The Ca atom splits into two sites at a distance of 0.161 nm apart, and it is coordinated with the three water molecules and the three oxygen atoms belonging to the framework. Al atoms preferably substitute a part of the Si atoms at the "bridging SiO4" tetrahedra rather than at those of the "chain middle group SiO4" tetrahedra.